Edited the main index.md serving as the homepage.

  - Moved releases section to top, and added `Downloads` to the title.
  
  - Added list entry and page (head.md) for downloading head of trunk
    archives.

Updated the localdoc processing to protect the new download page
`head.md` against erasure.

Edited the guide to for retrieval of sources to point out the quick
download links first, then ratchet up the complexity as less
information is known. More explicit repository links.

Tweaked the github README.md to use links to markdown files instead of
the removed html. Check after the next sync cycle if that works.

Regenerated the embedded documentation.