Removed custom bibtex style settings from _config.yml. Added documentation in apa.md about handling duplicate label warnings in APA references, including advice on adding letters to years in .bib files.
Removed local APA style extensions and related static files, updated _config.yml to use the sphinx_apa_references extension, and revised documentation in apa.md to reflect installation and usage of the new package. Added sphinx-apa-references to requirements.txt and references.bib now includes a multi-author example for APA citation demonstration.
Added new pages for Sphinx exercise and Sphinx proof extensions, including installation and usage instructions. Updated references, credits, and feature pages to reflect inclusion in TeachBooks Template and Favourites. Improved consistency of external links and badges, and updated submodules to latest versions.
