Port the site documentation from APT to Markdown

[slachiewicz]

Converts the 11 page(s) under src/site/apt to Markdown. APT is a format only
Doxia understands; Markdown is already supported by maven-site-plugin out of
the box, so no POM change is needed.

This PR targets master. The maven-resources-plugin-3.x maintenance line is covered by the sibling
PR #503, so the conversion lands on both lines and is not lost on the next
merge.

The pages were converted with doxia-converter and then cleaned up by hand:

• <<<code>>> becomes backticks, <<bold>> becomes **bold**, and
  {{{url}text}} becomes [text](url)
• the ASF licence 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 the page means to display rather than resolve is written
  ${esc.d}{...}

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

Part of the wider migration tracked in
apache/maven-doxia-converter#139

One thing worth a reviewer's eye, pre-existing on master only: in
examples/custom-resource-filters.apt.vm three paragraphs start in column 1,
which APT reads as a section title, so the current site renders them as
top-level headings rather than prose. The Markdown keeps that rendering so this
change stays a pure format migration and the page-by-page comparison comes out
empty. It is an authoring slip in the APT and is probably worth a separate
follow-up to turn those three headings back into paragraphs.