replace mkdocs with sphinx/myst-parser for HTML documentation generation

- fixes https://github.com/shaarli/Shaarli/issues/1451
- tools/.gitattributes: exclude doc/conf.py and doc/requirements.txt from zip exports
- tools/doc/sphinx: suppress myst.xref_missing warnings caused by https://github.com/executablebooks/MyST-Parser/issues/564
- dockerfile: use makefile/sphinx instead of mkdocs to build HTML documentation
- dockerfile: add bash to the docs build container (make: bash: No such file or directory)
- tools/doc/readthedocs: force use of python 3.11 (readthedocs ERROR: No matching distribution found for sphinx==7.1.0)
- tools/doc/readthedocs: add all required configuration variables https://docs.readthedocs.io/en/latest/config-file/v2.html#build-os
- tools/doc/readthedocs: override build commands to allow the source directory to be different from the conf.py directory (https://docs.readthedocs.io/en/stable/config-file/v2.html#build-commands, https://github.com/readthedocs/readthedocs.org/issues/1543)
- tools/doc/readthedocs: manually set output directory (readthedocs ERROR: No _readthedocs/html folder was created during this build.)
- doc: replace all references to mkdocs with sphinx

.gitattributes

@@ -40,6 +40,7 @@ Dockerfile*       export-ignore
 Doxyfile          export-ignore
 Makefile          export-ignore
 node_modules/     export-ignore
-mkdocs.yml        export-ignore
+doc/conf.py       export-ignore
+doc/requirements.txt export-ignore
 phpunit.xml       export-ignore
 tests/            export-ignore