feat: configure dune rules for site content with jon-site package
Create a separate jon-site package to build site content via odoc's
documentation stanza. This avoids index.mld basename conflicts with
the root package and enables hierarchical page names via glob_files_rec.
Changes:
- Add site/dune-project defining the jon-site package
- Update site/dune with documentation stanza using glob_files_rec for
blog, notebooks, reference, drafts, and static assets
- Move @children_order tags before headings (required by odoc)
- Fix cross-references from /site/ to /jon-site/ across all .mld files
- Update site/index.mld to use absolute page references
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>