The /docs directory in MediaWiki core contains various documentation files in plain text (.txt) format. Since Doxygen doesn't index txt files, these docs aren't included in the generated documentation, making them difficult to access. Following the implementation of T232104, Doxygen now indexes files in Markdown format, allowing us to pull these docs into https://doc.wikimedia.org by converting them to Markdown. See the discussion in T111298.
Convert the following files from .txt to .md, adding level 1 page titles and adjusting formatting to comply with Doxygen-supported Markdown: