Adds a Sphinx extension at docs/_source/_ext/makefile_targets.py that
parses the project Makefile's '## description' annotations and writes
docs/_source/api/makefile.rst at build time. Same convention 'make help'
uses, so the reference stays in sync with the source.
Generated page is grouped by target prefix (fetch-, ingest-, distill-,
docs-, etc.) and rendered as a list-table. Shows on RTD alongside the
autodoc API modules.
Generated file is gitignored — RTD regenerates on every build.
