Docs: Update "How to import private repositories" (Diátaxis)

[benjaoming]

This how-to needs love: https://docs.readthedocs.io/en/stable/guides/importing-private-repositories.html

• Update warning box
• Use tabbed interface
• Make difference between connected accounts with automatic configuration vs. manual configuration clear
• Add a screenshot to illustrate pages with SSH keys

Refs: #9747

[ericholscher]

Simple enough 👍

[benjaoming]

Had another read-through and didn't spot anything, neither! (it's been over a month, so I can say that I've managed to forget basically everything about writing this)