[docs] GitHub proxy admin guide and tsh reference

[greedy52]

related:

• GitHub proxy #48762

note that the guide for the old method is removed:

• https://goteleport.com/docs/admin-guides/management/guides/ssh-key-extensions/

[github-actions]

Amplify deployment status

Branch	Commit	Job ID	Status	Preview	Updated (UTC)
STeve/48762_docs	dfdce24	6	✅SUCCEED	steve-48762-docs	2025-01-07 19:57:40

[ptgott]

I would make the title more descriptive about what the integration entails, e.g., "Manage GitHub Access with Teleport" (depending on what's accurate here).

[greedy52]

not sure what's a good name here. i've changed it to Git Access with GitHub as we don't manage access yet. we only proxying git commands. the manage access part will come at a later time for another feature.