[Flang] Fix release blocker issue #46931 related to documentation.

Summary:
Fixes bug : https://bugs.llvm.org/show_bug.cgi?id=46931
This commit add a new flag -DLLVM_ENABLE_SPHINX=ON to cmake command to generate sphinx documentation,
along with new cmake targets `docs-flang-html`.
`ninja docs-flang-html` - generates sphinx documentation.
Generated release notes are present in <builddir>/tools/flang/docs/html/ folder.

Reviewers: richard.barton.arm, DavidTruby

Tags: #flang

Differential Revision: https://reviews.llvm.org/D85470

(cherry picked from commit 2fc86cc)

flang/docs/ReleaseNotes.rst

@@ -0,0 +1,96 @@
+========================================
+Flang 11.0.0 (In-Progress) Release Notes
+========================================
+
+.. contents::
+   :local:
+   :depth: 2
+
+.. warning::
+
+   These are in-progress notes for the upcoming LLVM 11.0.0 release.
+   Release notes for previous releases can be found on
+   `the Download Page <https://releases.llvm.org/download.html>`_.
+
+Introduction
+============
+
+This document contains the release notes for the Flang Fortran
+frontend, part of the LLVM Compiler Infrastructure, release 11.0.0. Here we
+describe the status of Flang in some detail, including major
+improvements from the previous release and new feature work. For the
+general LLVM release notes, see `the LLVM
+documentation <https://llvm.org/docs/ReleaseNotes.html>`_. All LLVM
+releases may be downloaded from the `LLVM releases web
+site <https://llvm.org/releases/>`_.
+
+Note that if you are reading this file from a Git checkout, this document
+applies to the *next* release, not
+the current one. To see the release notes for a specific release, please
+see the `releases page <https://llvm.org/releases/>`_.
+
+Known Issues
+============
+
+These are issues that couldn't be fixed before the release. See the bug reports for the latest status.
+
+- ...
+
+Introducing Flang
+=================
+
+Flang is LLVM's Fortran front end and is new for the LLVM 11 release.
+
+Flang is still a work in progress for this release and is included for
+experimentation and feedback.
+
+Flang status
+------------
+
+Flang is able to parse a comprehensive subset of the Fortran language
+and check it for correctness. Flang is not yet able to generate LLVM IR for
+the source code and thus is unable to compile a running binary. 
+
+Flang is able to unparse the input source code into a canonical form and emit
+it to allow testing. Flang can also invoke an external Fortran compiler on this
+canonical input.
+
+Flang's parser has comprehensive support for:
+- Fortran 2018
+- OpenMP 4.5
+- OpenACC 3.0
+
+Major missing features
+----------------------
+
+- Flang is not supported on Windows platforms.
+
+Using Flang
+===========
+
+Usage: ``flang hello.f90 -o hello.bin``
+
+Flang will parse the Fortran file ``hello.f90`` then unparse it to a canonical
+Fortran source file. Flang will then invoke an external Fortran compiler to
+compile this source file and link it, placing the resulting executable
+in ``hello.bin``.
+
+To specify the external Fortran compiler, set the ``F18_FC`` environment
+variable to the name of the compiler binary and ensure it is on your ``PATH``.
+The default value for ``F18_FC`` is ``gfortran``.
+
+When invoked with no source input, Flang will wait for input on standard in.
+When invoked in this way, Flang performs the same actions as if called with
+``-fdebug-measure-parse-tree -funparse`` and does not invoke ``F18_FC``.
+
+For a full list of options that Flang supports, run ``flang --help``.
+
+Additional Information
+======================
+
+Flang's documentation is located in the ``flang/docs/`` directory in
+the LLVM monorepo.
+
+If you have any questions or comments about Flang, please feel free to
+contact us via the `mailing
+list <https://lists.llvm.org/mailman/listinfo/flang-dev>`_.

flang/docs/_static/llvm.css

@@ -0,0 +1,112 @@
+/*
+ * LLVM documentation style sheet
+ */
+
+/* Common styles */
+.body { color: black; background: white; margin: 0 0 0 0 }
+
+/* No borders on image links */
+a:link img, a:visited img { border-style: none }
+
+address img { float: right; width: 88px; height: 31px; }
+address     { clear: right; }
+
+table       { text-align: center; border: 2px solid black;
+              border-collapse: collapse; margin-top: 1em; margin-left: 1em;
+              margin-right: 1em; margin-bottom: 1em; }
+tr, td      { border: 2px solid gray; padding: 4pt 4pt 2pt 2pt; }
+th          { border: 2px solid gray; font-weight: bold; font-size: 105%;
+              background: url("lines.gif");
+              font-family: "Georgia,Palatino,Times,Roman,SanSerif";
+              text-align: center; vertical-align: middle; }
+/*
+ * Documentation
+ */
+/* Common for title and header */
+.doc_title, .doc_section, .doc_subsection, h1, h2, h3 {
+  color: black; background: url("lines.gif");
+  font-family: "Georgia,Palatino,Times,Roman,SanSerif"; font-weight: bold;
+  border-width: 1px;
+  border-style: solid none solid none;
+  text-align: center;
+  vertical-align: middle;
+  padding-left: 8pt;
+  padding-top: 1px;
+  padding-bottom: 2px
+}
+
+h1, .doc_title, .title { text-align: left;   font-size: 25pt }
+
+h2, .doc_section   { text-align: center; font-size: 22pt;
+                     margin: 20pt 0pt 5pt 0pt; }
+
+h3, .doc_subsection { width: 75%;
+                      text-align: left;  font-size: 12pt;
+                      padding: 4pt 4pt 4pt 4pt;
+                      margin: 1.5em 0.5em 0.5em 0.5em }
+
+h4, .doc_subsubsection { margin: 2.0em 0.5em 0.5em 0.5em;
+                         font-weight: bold; font-style: oblique;
+                         border-bottom: 1px solid #999999; font-size: 12pt;
+                         width: 75%; }
+
+.doc_author     { text-align: left; font-weight: bold; padding-left: 20pt }
+.doc_text       { text-align: left; padding-left: 20pt; padding-right: 10pt }
+
+.doc_footer     { text-align: left; padding: 0 0 0 0 }
+
+.doc_hilite     { color: blue; font-weight: bold; }
+
+.doc_table      { text-align: center; width: 90%;
+                  padding: 1px 1px 1px 1px; border: 1px; }
+
+.doc_warning    { color: red; font-weight: bold }
+
+/* <div class="doc_code"> would use this class, and <div> adds more padding */
+.doc_code, .literal-block
+                { border: solid 1px gray; background: #eeeeee;
+                  margin: 0 1em 0 1em;
+                  padding: 0 1em 0 1em;
+                  display: table;
+                }
+
+blockquote pre {
+        padding: 1em 2em 1em 1em;
+        border: solid 1px gray;
+        background: #eeeeee;
+        margin: 0 1em 0 1em;
+        display: table;
+}
+
+h2+div, h2+p {text-align: left; padding-left: 20pt; padding-right: 10pt;}
+h3+div, h3+p {text-align: left; padding-left: 20pt; padding-right: 10pt;}
+h4+div, h4+p {text-align: left; padding-left: 20pt; padding-right: 10pt;}
+
+/* It is preferable to use <pre class="doc_code"> everywhere instead of the
+ * <div class="doc_code"><pre>...</ptr></div> construct.
+ *
+ * Once all docs use <pre> for code regions, this style can  be merged with the
+ * one above, and we can drop the [pre] qualifier.
+ */
+pre.doc_code, .literal-block { padding: 1em 2em 1em 1em }
+
+.doc_notes      { background: #fafafa; border: 1px solid #cecece;
+                  display: table; padding: 0 1em 0 .1em }
+
+table.layout    { text-align: left; border: none; border-collapse: collapse;
+                  padding: 4px 4px 4px 4px; }
+tr.layout, td.layout, td.left, td.right
+                { border: none; padding: 4pt 4pt 2pt 2pt; vertical-align: top; }
+td.left         { text-align: left }
+td.right        { text-align: right }
+th.layout       { border: none; font-weight: bold; font-size: 105%;
+                  text-align: center; vertical-align: middle; }
+
+/* Left align table cell */
+.td_left        { border: 2px solid gray; text-align: left; }
+
+/* ReST-specific */
+.title { margin-top: 0 }
+.topic-title{ display: none }
+div.contents ul { list-style-type: decimal }
+.toc-backref    { color: black; text-decoration: none; }

flang/docs/_themes/llvm-theme/layout.html

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+{#
+    sphinxdoc/layout.html
+    ~~~~~~~~~~~~~~~~~~~~~
+
+    Sphinx layout template for the sphinxdoc theme.
+
+    :copyright: Copyright 2007-2010 by the Sphinx team, see AUTHORS.
+    :license: BSD, see LICENSE for details.
+#}
+{% extends "basic/layout.html" %}
+
+{% block relbar1 %}
+<div class="logo">
+  <a href="{{ pathto('index') }}">
+    <img src="{{pathto("_static/logo.png", 1) }}"
+         alt="LLVM Logo" width="250" height="88"/></a>
+</div>
+{{ super() }}
+{% endblock %}
+
+{# put the sidebar before the body #}
+{% block sidebar1 %}{{ sidebar() }}{% endblock %}
+{% block sidebar2 %}{% endblock %}