site-builder: generate explicit odoc dune rules for the site
Replace the (documentation ...) stanza in site/dune with explicit
odoc compile → link → html-generate rules generated by a small OCaml
tool (site-builder/gen_rules.exe).
The generator walks site/ finding .mld files and static assets, then
emits two directory-target rules (compile+link → _odoc/, html-generate
+ support-files + assets → _html/) and a top-level @site alias.
Build the site with: dune build @site
Output lands in: _build/default/site/_html/
The committed site/dune.inc is kept in sync via dune build @runtest
(diff check against the generated output).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>