docs: untangle connector reference into reference, how-to guides and tutorials

[bajtos]

Quoting #5718 (comment)

Another concern is the connectors reference entry. It's a bit weird to see it under references because they are actually mixed with how tos and tutorials 🙈

Let's review the documentation for connectors and rework it into reference guides, how-to guides and tutorials.

This is a follow-up to #5718, see also #5549.

[dhmlau]

Closing in favor of #6091, #6092 and #6093