Initial version of sphinx documentation and...

- remove ubuntu 21.04 from CI (end of life)
- adjust matrix strategy for clang.yml
- Use lessons learned from RetroArcher on localize.yml, crowdin.yml, and locale.py
- Add end of life comments to Dockerfiles
- Adjust dependency order in Dockerfiles

docs/source/contributing/testing.rst

@@ -0,0 +1,42 @@
+:github_url: https://github.com/RetroArcher/RetroArcher/tree/nightly/docs/source/contributing/testing.rst
+
+Testing
+=======
+
+Clang Format
+------------
+Source code is tested against the `.clang-format` file for linting errors. The workflow file responsible for clang
+format testing is `.github/workflows/clang.yml`.
+
+Test clang-format locally.
+   .. Todo:: This documentation needs to be improved.
+
+   .. code-block:: bash
+
+      clang-format ...
+
+Sphinx
+------
+Sunshine uses `Sphinx <https://www.sphinx-doc.org/en/master/>`_ for documentation building. Sphinx is included
+in the `./scripts/requirements.txt` file. Python is required to build sphinx docs. Installation and setup of python
+will not be covered here.
+
+The config file for Sphinx is `docs/source/conf.py`. This is already included in the repo and should not be modified.
+
+Test with Sphinx
+   .. code-block:: bash
+
+      cd docs
+      make html
+
+   Alternatively
+
+   .. code-block:: bash
+
+      cd docs
+      sphinx-build -b html source build
+
+Unit Testing
+------------
+.. Todo:: Sunshine does not currently have any unit tests. If you would like to help us improve please get in contact
+   with us, or make a PR with suggested changes.