docs: document Custom UI comment and viewport APIs

[caio-pizzol]

Documents the Custom UI APIs that landed with the recent comment and viewport work.

• Adds concise docs for ui.comments.setActive, pendingSelection, ui.viewport.observe, ui.viewport.getScrollContainer, and body text-range getRect.
• Updates Custom UI demo overlays/popovers to remeasure through ui.viewport.observe instead of manual DOM listeners.
• Removes stale wording that said text-range getRect was deferred.

Verified:

• pnpm --prefix apps/docs check:em-dashes
• pnpm --prefix apps/docs check:imports
• pnpm --prefix apps/docs test:examples
• pnpm check:public:superdoc

[github-actions]

📖 Docs preview: https://superdoc-caio-docs-custom-ui-api-coverage.mintlify.app