llmstxt.org does not serve the spec's .html.md URLs for its own pages

[keplervital]

The proposal states that markdown versions of pages live at the page URL with .md appended, and that "URLs without file names should append index.html.md instead."

llmstxt.org itself only does this for the notebook-built pages:

curl -o /dev/null -sw '%{http_code}\n' https://llmstxt.org/index.html.md    # 404
curl -o /dev/null -sw '%{http_code}\n' https://llmstxt.org/nbdev.html.md    # 404
curl -o /dev/null -sw '%{http_code}\n' https://llmstxt.org/domains.html.md  # 404
curl -o /dev/null -sw '%{http_code}\n' https://llmstxt.org/ed.html.md       # 404
curl -o /dev/null -sw '%{http_code}\n' https://llmstxt.org/intro.html.md    # 200
curl -o /dev/null -sw '%{http_code}\n' https://llmstxt.org/core.html.md     # 200

In particular the homepage is the exact "URL without a file name" case from the spec text, but /index.html.md is 404 and the markdown only exists at /index.md.

Cause: keep-md only produces .html.md files for the executed notebooks. The qmd and md pages only get the commonmark render, named {stem}.md (or {stem}-commonmark.md when the source itself is a .md file).

Fix available in #134: a small post-render script that adds the .html.md names while keeping every existing URL (including /index.md, which llms.txt links on purpose) working unchanged.