Review and improve Git proxy getting started and config documentation

[kriswest]

Is your feature request related to a problem? Please describe.
The Git Proxy documentation is incomplete and contains incorrect information (e.g. Pre-receive hooks are not actually a thing).

Describe the solution you'd like
The documentation needs a review, identification of gaps and issues raising for specific introductory content to be written.

Review criteria (PR template) should be updated to prompt reviewers to check for config schema and introductory docs on any new config features.

Describe alternatives you've considered
Continuing as we are is not viable as it will reduce uptake of Git Proxy by making setup harder to understand and the available feature set harder to figure out.