Port the site documentation from APT to Markdown

[slachiewicz]

Converts the 76 pages 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.

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.

The four modules under its/ could not be built in my environment, so their
five pages are converted but unverified. Their %{snippet} macros were checked
by hand against the originals.

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

[gnodet]

Great work on this APT-to-Markdown migration! The conversion is well-executed across most of the 76 files — links, tables, macros, license headers, and Velocity template handling are all cleanly converted.

A handful of conversion artifacts worth addressing before merge:

Medium:

1. Spurious HTML anchors in property references — ${maven.home} became $<a id="maven.home"></a>maven.home in configuration-management.md (6 occurrences) and getting-to-container-configured-mojos.md (1 occurrence). These should remain as literal ${maven.home} or in backticks.

2. Lost structure in Properties subsection — In configuration-management.md, the "Properties" subsection heading became plain text and three property definitions collapsed into a single paragraph, losing readability.

3. XML elements rendered as emphasis — <developerConnection/> became _developerConnection/_ (italic) in configuration-management.md (3 occurrences). Should use backticks: `<developerConnection/>`.

Low:
4. Leading spaces in link text — [ Settings Reference](...) instead of [Settings Reference](...) in maven-settings/index.md (5 links), maven-compat/index.md (1 link), and artifact-handlers.md (1 link).

5. unknown code fence language — 12 code fences across 6 files use ```unknown as language identifier. Should use bare ``` or ```text.

This review was generated by an AI agent and may contain inaccuracies. Please verify all suggestions before applying.

Claude Code on behalf of gnodet

[gnodet]

All 5 previously identified conversion artifacts have been fixed in the latest commits:

1. ✅ Spurious HTML anchors in property references — now properly rendered in backticks
2. ✅ Lost structure in Properties subsection — proper heading and fenced code block
3. ✅ XML elements rendered as emphasis — now using backticks
4. ✅ Leading spaces in link text — cleaned up
5. ✅ Unknown code fence language — removed

The Velocity template handling in .md.vm files is well done, and the test file updates correctly parse the new Markdown table format.

This review was generated by an AI agent and may contain inaccuracies. Please verify all suggestions before applying.

Claude Code on behalf of gnodet