docs: resolve guide and doc items in the router - #33895

`GuideViewer` and `ComponentViewer` looked up their item from the route
params themselves, and called `router.navigate` when nothing matched.
That meant the component was created and rendered before the redirect,
and the redirect started a second navigation.

This moves the lookup into route resolvers. When an item is missing, the
resolver returns a `RedirectCommand`, so the router redirects as part of
the same navigation, before the component is created. When it's found,
the item reaches the component as an input through
`withComponentInputBinding()`.

Changes:
- Add `guideResolver` on the `guide/:id` route. It resolves the guide or
  redirects to `/guides`.
- Add `docItemResolver` on the `:section/:id` route. It resolves the doc
  item or redirects to the section.
- `GuideViewer`: replace the `id` input and lookup with a required
  `guide` input, and drop the `Router` and `GuideItems` injections.
- `ComponentViewer`: replace the `id` and `section` inputs and the async
  lookup with a required `docItem` input. `componentDocItem` is now
  `toObservable(docItem)`, which replays the latest item like the old
  `ReplaySubject`, so the child pages are unchanged. The tabs in
  `sections` become a `computed`, so they're always in the same order
  instead of depending on the order of previous add/delete calls.

Tests:
- Run the `GuideViewer` and `ComponentViewer` specs against routes that
  use the resolvers, so they cover both the resolved inputs and the
  redirects.
- Add a test for the tabs shown for a doc item.