availability updates for source and documentation

[Sitaram Chamarty]

1. Gitolite is now available from
https://codeberg.org/sitaramc/gitolite also.

2. Documentation is now "self-hostable". That is, you can get
a ZIP file of the latest documentation from
https://codeberg.org/sitaramc/gitolite-doc/releases, unzip
it, and point your browser at index.html.

There were a lot of changes involved in getting this done,
mainly because I had not realised github was silently
fixing broken URLs. Ouch. (And no thanks!)

Plus, the doc build process is no longer dependent on some
quirky/kludgey scripts that I really can't see airing in
public :-) Anyone with pandoc, mkdocs (+ material theme),
vim, and xterm can build the documentation. As a bonus, search is *much* nicer now.

Details are available in docs/build-docs.md in the
gitolite-doc repository (online version at
https://gitolite.com/gitolite/build-docs.html)

Sitaram