Port the site documentation from APT to Markdown

[slachiewicz]

The maven-compiler-plugin-3.x maintenance line still carries its site documentation in APT, so a release from it would publish the old pages while the default branch serves Markdown. This brings the two into line.

Two commits: a pure git mv so that git log --follow and git blame still reach each page's history, then the conversion. Please merge or rebase rather than squash — squashing collapses the rename.

Things that needed doing by hand, each found by comparing generated HTML rather than by reading the converted source:

• Velocity reads ## as a line comment, so in a page that keeps its .vm suffix every heading below level one would be silently deleted. Level two uses a setext underline, deeper ones an unparsed #[[ ]]# block.
• Where APT wrote $\{foo\} for a reference it means to display, its parser strips the backslashes, so the converted page carried a live reference. Those are now ${esc.d}{foo} — in .vm pages only, since a plain .md is never run through Velocity and ${esc.d} would render literally there.
• A few things APT renders literally that Markdown reinterprets: a double hyphen (the typographic extension folds it into an en dash), an asterisk mid-line, and an emphasis run that ends in *.

Verified by building the site before and after and comparing all 27 generated pages: every one is identical in its title, author and date metadata, its visible text and every link target.