Follow-up to #69205. After switching adev's COEP to `credentialless`, the
cross-origin YouTube iframe in `<docs-video>` loads in Chromium and Safari but
not Firefox, whose `credentialless` policy does not extend to nested frames. The
result was a COEP error screen instead of the player.
Render `<docs-video>` as a lightweight thumbnail facade instead of embedding the
iframe directly. The thumbnail is a cross-origin subresource, so it loads under
`credentialless` in every browser. `DocViewer` then upgrades the facade to the
inline player on hydration in browsers that can load the embed (Chromium,
Safari), preserving the previous behavior there. On Firefox the facade stays a
plain link that opens the video on YouTube (with autoplay), which replaces the
error screen.
The thumbnail uses `maxresdefault` and falls back to `hqdefault` when a video
has no max-resolution image.
To increase the ease of development we are moving @angular/docs into the adev directory within this repo. While
we are doing this to improve our development experience in the short term, efforts are also in place
to maintain a division between this @angular/docs (shared) code and adev itself, so that it can be extracted
back out in the future when components is ready to leverage it as well.
PR Close#57132
Kam