Convert the demo archetypes' pages to Markdown

[slachiewicz]

Extends the Markdown-only treatment to the two sibling demo archetypes. Both handed a generated
project an APT src/site while shipping markdown.md and markdown-velocity.md.vm beside it,
so a new project started life with two documentation formats and no reason to prefer either.

Independent of #144 — different files, no overlap.

Per-file judgement, as asked

file	call	why
plugin-site/…/apt/index.apt	convert	the sample plugin's front page — real content
plugin-site/…/apt/usage.apt	convert	a usage page the user fills in — real content
plugin-site/…/apt/examples/example.apt	convert	a worked SMTP-authentication example — real content
site-simple/…/apt/index.apt	convert	the generated project's landing page — real content
—	delete: none	none of the four is a format demonstration

That is the difference from maven-archetype-site in #144, where format.apt existed only to
show what APT looks like and has no Markdown equivalent worth writing. Here every page carries
content that survives translation, so nothing is deleted.

Neither site descriptor needs a change — checked, not assumed

Conversion does not rename a generated page: index.apt→index.md still produces index.html.
Every href in both menus resolves exactly as before, and no page is removed, so there is nothing
to prune. Both menus are left alone.

For the record, entries like faq.html, plugin-info.html, changes-report.html and
jira-report.html point at pages the template does not contain — they are placeholders for the
user to enable, not breakage, and predate this change.

The reference fixture moves in lockstep

site-simple/src/test/resources/projects/it-basic/reference/ is the expected generated output.
Its src/site/apt/index.apt is now src/site/markdown/index.md, byte-identical to what the
archetype produces. The IT compares the two, so this is a real regression test rather than a
copy that has to be kept in step by hand.

Verification

Both archetypes' ITs (archetype:integration-test, goal clean site) run before and after —
they generate a project and build its site:

• both pass, including site-simple's reference comparison
• the same pages are produced on both sides
• every generated page is identical in visible text, <title>/author/date metadata,
  structural tag sequence, and anchor id set

The only diff anywhere is the absolute build path inside distribution-management.html, which
differs because the two runs were in different directories.

Two things established by measurement, worth recording

Archetype filtering does not eat ATX headings. Both descriptors ship src/site as a
<fileSet> without filtered="false", so I checked whether ## would be swallowed as a Velocity
line comment at generate time. It is not: ### Subsection in the existing markdown.md
survives generation and renders as <h3>, and ${artifactId} in the generated site.xml is not
substituted either — src/site is effectively unfiltered. So these pages keep ordinary ATX
headings; no setext workaround is needed for .md template content.

Metadata is taken from each page's own APT header, not harmonised across siblings — the four
pages declare different authors and dates and keep them. site-simple/index.md keeps its
placeholder Title Here / Author Here / Date Here (AAAA-MM-DD), which is what the template
is for.

Local runs are JDK 21 / Maven 3.9.16 / macOS only; CI is the real check.

Generated with assistance from Claude Opus 5.