Docs: Add "Git provider account connection" feature description (#10442)
* Add "Git provider account connection" to docs: Scissor out and update work from https://github.com/benjaoming/readthedocs.org/pull/5
* Turn bullet list "wall of text" into a description list
* Copy edit the docs page.
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Co-authored-by: Eric Holscher <25510+ericholscher@users.noreply.github.com>