[RazorDocs] Add deep-page wayfinding with TOC, related links, and next steps

[akrock]

Summary

Add local wayfinding patterns to deep RazorDocs pages so readers can orient quickly and continue without backtracking.

Scope

• Add a page summary or purpose block near the top of substantial pages.
• Show the page type prominently.
• Add an on-page table of contents when heading depth warrants it.
• Add related links and next-step suggestions from metadata.
• Add previous and next navigation for ordered guide and example sequences.
• Expand the sidebar around the active page so nearby siblings and parents are visible.

Acceptance Criteria

• Deep pages show a TOC when content depth is meaningful.
• Related links and next steps render from metadata.
• Ordered guide and example sequences can show previous and next navigation.
• The active section is visible in the sidebar.
• Tests cover TOC extraction and related-link rendering.