Expand and update 'Configuring GitHub repositories'

[DavidBiddle]

We have a very short paragraph about configuring Github repos on the 'Use Github' page. It includes this text:

Consider protecting your main branch to prevent changes being committed without a suitable review.

We should consider:

1. Whether we foresee any circumstance where we don't want protections on the main branch
2. Whether we want to recommend any specific protections on the main branch

and update this paragraph accordingly.

[andyloughran]

There seem to be a number or organisaitonal controls that can help evaluate whether repos within that organisation are setup in accordance with the guidance published here.

[timblair]

Short term review of existing controls (and at what level they apply: enterprise, org, repo) and appropriate updates. Future work (including overall service ownership) will be picked up by Engineering Enablement, which will probably require a wider review.