Port the site documentation from APT to Markdown

9 page(s) converted with doxia-converter and then cleaned up by hand:

- <<<code>>> becomes backticks, <<bold>> becomes **bold**, {{{url}text}}
  becomes [text](url)
- the ASF license header becomes a single block HTML comment
- a page whose only Velocity reference was ${project.name} is now a plain
  .md with the title spelled out
- a page that keeps a reference stays a .vm. Velocity reads ## as a line
  comment and would silently swallow every ATX heading below level one, so
  subsections use setext underlines and anything deeper is wrapped in
  #[[ ... ]]#
- references APT rendered literally are escaped, so they do not resolve

Verified by building the site before and after and comparing every
generated page: the visible text and the link targets are unchanged apart
from <b>/<i> becoming <strong>/<em> and quotes in prose picking up the
Markdown module's typographic substitution.