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/toc.rst

@@ -0,0 +1,27 @@
+.. toctree::
+   :maxdepth: 2
+   :caption: About
+
+   about/overview
+   about/installation
+   about/docker
+   about/third_party_packages
+   about/usage
+   about/advanced_usage
+
+.. toctree::
+   :maxdepth: 2
+   :caption: Build
+
+   building/build
+   building/linux
+   building/macos
+   building/windows
+
+.. toctree::
+   :maxdepth: 2
+   :caption: Contributing
+
+   contributing/contributing
+   contributing/localization
+   contributing/testing