docs: Rework connector reference contents

[dhmlau]

Originated from #5786: untangle connector reference into reference, how-to guides and tutorials.

Since this will involve quite a lot of effort, I'm creating an epic per @bajtos' suggestion.

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[stale]

This issue has been closed due to continued inactivity. Thank you for your understanding. If you believe this to be in error, please contact one of the code owners, listed in the CODEOWNERS file at the top-level of this repository.