Make the site archetype a Markdown-only demonstration

[slachiewicz]

Makes maven-archetype-site a Markdown-only demonstration: a project generated from it no
longer starts life with four documentation formats to choose between.

This is a product decision about the archetype, not a mechanical conversion — a reviewer seeing
faq.fml deleted should know it is deliberate. The archetype should teach the format the project
recommends rather than preserve one sample per legacy format.

Stacked on #143 and branched from it, because it edits two pages that #143 converts. Merge
#143 first; this diff will then reduce to its own three commits. Merge or rebase, don't squash.

What changes

Converted, not deleted — src/site/apt/index.apt and src/site/fr/apt/index.apt become
markdown/index.md and fr/markdown/index.md. They have to survive: it-basic/verify.groovy
asserts target/site/en/index.html and target/site/fr/index.html exist, and these generate them.

Deleted (6) — apt/format.apt, fml/faq.fml, xdoc/xdoc.xml and the three fr/
equivalents.

Kept — markdown/markdown.md, markdown/markdown-velocity.md.vm and both fr/ equivalents.

Both site descriptors lose the three menu entries whose pages are gone; leaving them would
generate a project whose menu 404s on first build. The remaining item names still say "Markdown",
which stays accurate and describes what the example demonstrates — I did not rename them.

Two pages that described the archetype are updated: this module's own index (its directory
tree listed every deleted file) and the aggregator's table.

The archetype descriptor needs no change — it ships src/site as a whole-directory
<fileSet> rather than listing files, so removals cannot break it. Checked specifically.

Verification

Ran the archetype IT (archetype:integration-test, goal clean site), which generates a project
and builds its site, on this branch and on pristine master:

• IT passes, both index.html assertions included
• generated pages: exactly fr/faq.html, fr/format.html and fr/xdoc.html disappear
• every remaining page differs only by the three removed menu entries — verified page by page
• the converted fr/index.html is otherwise byte-identical in visible text and metadata

A pre-existing bug this uncovered — worth its own issue, not fixed here

The generated project sets <locales>en,fr</locales>. The non-localized src/site/** content
is never rendered at all. On current master, a freshly generated project produces:

• en/index.html — the auto-generated About report, not the archetype's index page
• fr/index.html — the archetype's real index page
• the five sample pages only ever appear under fr/

so the English menu links to format.html, faq.html, markdown.html, markdown-velocity.html
and xdoc.html, and all five 404. That is today's behaviour, unchanged by this PR; this
change happens to reduce the broken English links from five to two, but does not fix the cause.

It also means the English markdown/index.md in this PR is not exercised by any build — the
French one is what the IT renders and what I verified. Flagging that rather than implying both
sides are covered.

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

Generated with assistance from Claude Opus 5.