According to the docs, Writing-your-docs:Index-pages, README.md will render into index.html. There is no mention of readme.md. The implementation reflects this behavior:

But the check in get_files() ignores casing:

I believe it should be:

if filename == 'README.md' and 'index.md' in filenames:
    log.warning(f"Both index.md and README.md found. Skipping README.md from {source_dir}")