Lead with Markdown in the site and plugin documentation guides

[slachiewicz]

Sits on top of #1632 — the first two commits here are that PR, since the plugin documentation guide has to be Markdown before its content can be rewritten. Review the last commit only; once #1632 merges this will rebase down to two files.

Two pieces of normative guidance still point new documentation at APT:

guides/mini/guide-site.md lists apt first and calls it the format "like this one" — which stopped being true when that guide itself became Markdown. It now lists markdown first, says plainly that new documentation should use it, and describes APT as a format that predates Maven's Markdown support: still supported, but the Maven project has moved its own documentation off it. Adds a pointer to doxia-converter for anyone converting an existing page.

guides/development/guide-plugin-documentation is the canonical instruction for documenting a plugin, and it gave a full APT template for src/site/apt/index.apt — so an author following it started in APT. It now shows the same page as src/site/markdown/index.md, with the YAML front matter that carries the title, author and date, plus a note about naming the file index.md.vm and keeping headings away from ## if the page needs Velocity.

One thing worth seeing: that guide is itself a Velocity template, and Velocity reads ## as a line comment even inside a fenced code block. The first version of this change silently lost ## Goals Overview and ## Usage from the rendered sample. It is now wrapped in #[[ ]]#. That only showed up because the built page was checked rather than the source.

Verified by building the site: the sample renders all its headings and no #[[ markup leaks into the page.

[elharo]

is --> are

[slachiewicz]

This is pure apt to Markdown conversion and now with Your previous commits I have conflicts