Add a View RDF link to the page-tools sidebar

[JeroenDeDauw]

The per-page RDF export (GET /rest.php/neowiki/v0/page/{pageId}/rdf) was only
reachable by hand-constructing the URL. Add a "View RDF" link to the NeoWiki
page-tools sidebar section, next to "View JSON", pointing at the current page's
Turtle export.

It is shown only when the page has Subjects, so it never points at the
endpoint's 404-on-empty response. It uses ?format=turtle so the result renders
as readable text in the browser instead of downloading a TriG file, and targets
the native projection. Per-ontology projection links and a per-subject RDF
endpoint (#1087) are out of scope here.

Relates to #1023

_{AI-authored — Claude Code, Opus 4.8 (max); @JeroenDeDauw asked for this sidebar link and to build it via the fix-issue workflow (no revisions during implementation); not yet human-reviewed; new PageToolsBuilder tests + full make cs (phpcs + phpstan) green, and verified live on the demo wiki — "View RDF" renders on a Subject page and its target returns text/turtle, and is absent on a Subject-less page. Full local PHPUnit is green apart from two pre-existing SPARQL end-to-end tests that need the QLever service (absent from this dev stack, unrelated to this change); CI covers them.}

[JeroenDeDauw]