Document internal architecture, contributing & hacking on Git Proxy

[coopernetes]

A follow-up from #425 and document the current architecture of Git Proxy in preparation for a full 2.0 release.

Some details which should be included:

• Architecture & internals
• Extending Git Proxy (plugins, already documented in feat: use native node loading for plugins + docs #638 and just needs to be validated against main for v2.0)
• Deploying Git Proxy into production