A first pass at pruning the Synapse README

[DMRobertson]

Commit-by-commit reviewable.

Closes #13486 (maybe?)

• Move some troubleshooting to the Admin FAQ
• Don't try to describe Matrix
• Don't duplicate the development quick start from docs. (To the reviewer: be convinced that the deleted content has an equivalent in the contributing guide)
• Rearrange the hierarchy of sections
• A few drive-by tweaks to prose.

Wasn't sure how to handle the identity server section. I don't think it really belongs here, but I don't want to rip it out and disregard users' privacy concerns.

[H-Shay]

I made some comments but am chucking this back into the queue to let others have their say. Overall I think it looks good, thank you so much for doing this!

[DMRobertson]

See rendered readme and the rendered admin faq.

[DMRobertson]

Thank you both!