Move docs to readthedocs (#899)

Official docs is now hosted on the readthedocs.
https://eventlet.readthedocs.io/en/latest/
Only branches or tags containing the previous
readthedocs setup are able to been displayed.

For now only the master branch contains the right setup.

Historically, the docs was hosted on gh-pages. This
previous setup only host the last built version.
No parallel versions of the docs were possible.

This new way of documenting eventlet allow us to
parallelize versions. That will be better to
reflect deprecations, removal, or additions.

Now remain a bit of work:
- make other maintainers readthedocs admin
- transfer eventlet.net to readthedocs
- disable gh-pages

README.rst

@@ -56,9 +56,9 @@ It's easy to get started using Eventlet, and easy to convert existing
 applications to use it.  Start off by looking at the `examples`_,
 `common design patterns`_, and the list of `basic API primitives`_.
 
-.. _examples: http://eventlet.net/doc/examples.html
-.. _common design patterns: http://eventlet.net/doc/design_patterns.html
-.. _basic API primitives: http://eventlet.net/doc/basic_usage.html
+.. _examples: https://eventlet.readthedocs.io/en/latest/examples.html
+.. _common design patterns: https://eventlet.readthedocs.io/en/latest/design_patterns.html
+.. _basic API primitives: https://eventlet.readthedocs.io/en/latest/basic_usage.html
 
 
 Getting Eventlet
@@ -76,12 +76,11 @@ To install latest development version once::
 Building the Docs Locally
 =========================
 
-To build a complete set of HTML documentation, you must have Sphinx, which can be found at http://sphinx.pocoo.org/ (or installed with `pip install Sphinx`)::
+To build a complete set of HTML documentation::
 
-  cd doc
-  make html
+  tox -e docs
 
-The built html files can be found in doc/_build/html afterward.
+The built html files can be found in doc/build/html afterward.
 
 Supported Python versions
 =========================

pyproject.toml

@@ -48,6 +48,7 @@ Homepage = "https://github.com/eventlet/eventlet"
 History = "https://github.com/eventlet/eventlet/blob/master/NEWS"
 Tracker = "https://github.com/eventlet/eventlet/issues"
 Source = "https://github.com/eventlet/eventlet"
+Documentation = "https://eventlet.readthedocs.io/"
 
 [project.optional-dependencies]
 dev = ["black", "isort", "pip-tools", "build", "twine", "pre-commit", "commitizen"]