Enhance Bilibili video support (redo of #2522) (#2599)

* Enhance support for bilibili videos in responsive video helper, and add corresponding doc

* Apply @iBug's review in mmistakes/minimal-mistakes#2522

* Fix danmaku in page hero video

* Update video to use case..when for iframe src

* Update CHANGELOG and history

Co-authored-by: Anran <51769728+ALeafWolf@users.noreply.github.com>
Co-authored-by: Michael Rose <mmistakes@users.noreply.github.com>

Merge branch 'release/4.2.0' into develop

Merge branch 'release/4.2.0'

Rename `responsive_video` helper to `video`

Add page hero video for vimeo/youtube using embedresponsively.com code (#788)