4.3 docs

[bastianallgeier]

I'm afraid, the Docs section in Notion was pretty rough. We probably should also add docs for:

• disablepictureinpicture here https://getkirby.com/docs/reference/text/kirbytags/video
• Plugin registration with named arguments

[distantnative]

I left the plugin registration out, I was afraid that it causes quite some confusion etc. So thought about making the switch in the docs only in v5?