docs: update guide viewer title when route params change - #33878

`GuideViewer` subscribed to the route params to resolve the current guide,
but only set the page title once in `ngOnInit`. Since the router reuses the
component when only the `:id` param changes, navigating from one guide to
another in-app would update the rendered guide while leaving the browser
title pointing at the previous one.

This isn't reachable today because guide-to-guide links in the markdown are
absolute URLs that trigger a full page load, but it would surface as soon
as an in-app link between guides is added.

Changes:
- Set the page title inside the params subscription, next to where the
  guide is resolved, so both stay in sync on every param emission.
- Remove the now-redundant `ngOnInit` and the `OnInit` interface.
- Tie the params subscription to the component's lifetime with
  `takeUntilDestroyed()`, matching the pattern used in `ComponentViewer`.
- Switch the spec's mocked `params` to a `BehaviorSubject` and add tests
  covering the initial title and the title after a param change.