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
  #[[ ... ]]#
- a reference APT rendered literally is escaped with ${esc.d}, which always
  yields a literal dollar; a backslash only works when the reference itself
  resolves

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 and ellipses in prose picking up
the Markdown module's typographic substitution.