Allow custom tags in .mld pages
Previously, custom extension tags like @admonition.note and @rfc were
only allowed in OCaml documentation comments, not in standalone .mld
pages. This made it impossible to demonstrate extensions in their own
documentation.
Change tags_allowed from false to true when parsing .mld page content,
enabling extension tags to work in standalone documentation pages.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>