docs: add style guidelines for contributing to the documentation

Add a set of recommendations on style and formatting for contributing to
Cilium's documentation. The current guidelines are mostly about RST
formatting for now, but can be extended with whatever directions may
help produce a better documentation in the future.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>

Documentation/contributing/development/documentation.rst

@@ -0,0 +1,272 @@
+.. only:: not (epub or latex or html)
+
+    WARNING: You are looking at unreleased Cilium documentation.
+    Please use the official rendered version released here:
+    https://docs.cilium.io
+
+*******************
+Documentation Style
+*******************
+
+.. |RST| replace:: reStructuredText
+
+Here are some guidelines and best practices for contributing to Cilium's
+documentation. They have several objectives:
+
+- Ensure that the documentation is rendered in the best possible way (in
+  particular for code blocks).
+
+- Make the documentation easy to maintain and extend.
+
+- Help keep a consistent style throughout the documentation.
+
+See also :ref:`the documentation for testing <testing-documentation>` for
+instructions on how to preview documentation changes.
+
+Header
+------
+
+Use the following header when adding new files to the Documentation.
+
+.. code-block:: rst
+
+  .. only:: not (epub or latex or html)
+
+          WARNING: You are looking at unreleased Cilium documentation.
+          Please use the official rendered version released here:
+          https://docs.cilium.io
+
+One exception is |RST| fragments that are supposed to be sourced from other
+documentation files. Those do not need this header.
+
+Titles
+------
+
+Prefer title case (capital letters on most words of the title) rather than
+sentence case for titles.
+See this link if necessary: https://titlecaseconverter.com/.
+
+Body
+----
+
+Wrap the lines for long sentences or paragraphs. There is no fixed convention
+on the length of lines, but targeting a width of about 80 characters should be
+safe in most circumstances.
+
+Code Blocks
+-----------
+
+Code snippets and other literal blocks usually fall under one of those three
+categories:
+
+- They contain `substitution references`_ (for example: ``|SCM_WEB|``). In that
+  case, always use the ``.. parsed-literal`` directive, otherwise the token
+  will not be substituted.
+
+  Prefer:
+
+  .. code-block:: rst
+
+    .. parsed-literal::
+
+        $ kubectl create -f \ |SCM_WEB|\/examples/minikube/http-sw-app.yaml
+
+
+  Avoid:
+
+  .. code-block:: rst
+
+    .. code-block:: shell-session
+
+        $ kubectl create -f \ |SCM_WEB|\/examples/minikube/http-sw-app.yaml
+
+- If the text is not a code snippet, but just some fragment that should be
+  printed verbatim (for example, the unstructured output of a shell command),
+  use the marker for `literal blocks`_ (``::``).
+
+  Prefer:
+
+  .. code-block:: rst
+
+    See the output in ``dmesg``:
+
+    ::
+
+        [ 3389.935842] flen=6 proglen=70 pass=3 image=ffffffffa0069c8f from=tcpdump pid=20583
+        [ 3389.935847] JIT code: 00000000: 55 48 89 e5 48 83 ec 60 48 89 5d f8 44 8b 4f 68
+
+    See more output in ``dmesg``::
+
+        [ 3389.935849] JIT code: 00000010: 44 2b 4f 6c 4c 8b 87 d8 00 00 00 be 0c 00 00 00
+        [ 3389.935850] JIT code: 00000020: e8 1d 94 ff e0 3d 00 08 00 00 75 16 be 17 00 00
+
+  Avoid:
+
+  .. code-block:: rst
+
+    See the output in ``dmesg``:
+
+    .. parsed-literal::
+
+        [ 3389.935842] flen=6 proglen=70 pass=3 image=ffffffffa0069c8f from=tcpdump pid=20583
+        [ 3389.935847] JIT code: 00000000: 55 48 89 e5 48 83 ec 60 48 89 5d f8 44 8b 4f 68
+
+  The reason is that because these snippets contain no code, there is no need
+  to mark them as code or parsed literals. The former would tell Sphinx to
+  attempt to apply syntax highlight, the second would tell it to look for |RST|
+  markup to parse in the block.
+
+- If the text contained code or structured output, use the ``.. code-block``
+  directive. Do *not* use the ``.. code`` directive, which is slightly less
+  flexible.
+
+  Prefer:
+
+  .. code-block:: rst
+
+    .. code-block:: shell-session
+
+        $ ls
+        cilium
+        $ cd cilium/
+
+  Avoid:
+
+  .. code-block:: rst
+
+    .. parsed-literal::
+
+        $ ls
+        cilium
+        $ cd cilium/
+
+    .. code-block:: bash
+
+        $ ls
+        cilium
+        $ cd cilium/
+
+    .. code-block:: shell-session
+
+        ls
+        cilium
+        cd cilium/
+
+  The ``.. code-block`` directive should always take a language name as
+  argument, for example: ``.. code-block:: yaml`` or ``.. code-block::
+  shell-session``. The use of ``bash`` is possible but should be limited to
+  Bash scripts. For any listing of shell commands, and in particular if the
+  snippet mixes commands and their output, use ``shell-session``, which will
+  bring the best coloration and may trigger the generation of the ``Copy
+  commands`` button.
+
+For snippets containing shell commands, in particular if they also contain the
+output for those commands, use prompt symbols to prefix the commands. Use ``$``
+for commands to run as a normal user, and ``#`` for commands to run with
+administrator privileges. You may use ``sudo`` as an alternative way to mark
+commands to run with privileges.
+
+.. _substitution references: https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#substitution-references
+.. _literal blocks: https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#literal-blocks
+
+Links
+-----
+
+- Avoid using `embedded URIs`_ (```... <...>`__``), which make the document
+  harder to read when looking at the source code of the documentation. Prefer
+  to use `block-level hyperlink targets`_ (where the URI is not written
+  directly in the sentence in the |RST| file, below the paragraph).
+
+  Prefer:
+
+  .. code-block:: rst
+
+    See the `documentation for Cilium`_.
+
+    Here is another link to `the same documentation <cilium documentation>`_.
+
+    .. _documentation for Cilium:
+    .. _cilium documentation: https://docs.cilium.io/en/latest/
+
+  Avoid:
+
+  .. code-block:: rst
+
+    See the `documentation for Cilium <https://docs.cilium.io/en/latest/>__`.
+
+- If using embedded URIs, use anonymous hyperlinks (```... <...>`__`` with two
+  underscores, see the documentation for `embedded URIs`_) instead of named
+  references (```... <...>`_``, note the single underscore).
+
+  Prefer (but see previous item):
+
+  .. code-block:: rst
+
+    See the `documentation for Cilium <https://docs.cilium.io/en/latest/>__`.
+
+  Avoid:
+
+  .. code-block:: rst
+
+    See the `documentation for Cilium <https://docs.cilium.io/en/latest/>_`.
+
+.. _embedded URIs: https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#embedded-uris-and-aliases
+.. _block-level hyperlink targets: https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#hyperlink-targets
+
+Lists
+-----
+
+- Left-align the body of a list item with the text on the first line, after the 
+  item symbol.
+
+  Prefer:
+
+  .. code-block:: rst
+
+    - The text in this item
+      wraps of several lines,
+      with consistent indentation.
+
+  Avoid:
+
+  .. code-block:: rst
+
+    - The text in this item
+        wraps on several lines
+        and the indent is not consistent
+        with the first line.
+
+- For enumerated lists, prefer auto-numbering with the ``#.`` marker rather
+  than manually numbering the sections.
+
+  Prefer:
+
+  .. code-block:: rst
+
+    #. First item
+    #. Second item
+
+  Avoid:
+
+  .. code-block:: rst
+
+    1. First item
+    2. Second item
+
+Roles
+-----
+
+- We have a dedicated role for referencing Cilium GitHub issues, to reference
+  them in a consistent fashion. Use it when relevant.
+
+  Prefer:
+
+  .. code-block:: rst
+
+    See :gh-issue:`1234`.
+
+  Avoid:
+
+  .. code-block:: rst
+
+    See `this GitHub issue <https://github.com/cilium/cilium/issues/1234>`__.

Documentation/contributing/development/index.rst

@@ -24,3 +24,4 @@ understand the workflow of getting these changes reviewed and merged upstream.
    codeoverview
    debugging
    hubble
+   documentation

Documentation/contributing/testing/documentation.rst

@@ -4,6 +4,8 @@
     Please use the official rendered version released here:
     https://docs.cilium.io
 
+.. _testing-documentation:
+
 Documentation
 =============
 

Documentation/spelling_wordlist.txt

@@ -722,6 +722,7 @@ repos
 reproducibility
 requireIPv
 resourceQuotas
+reStructuredText
 retpoline
 retransmission
 retrigger