docs: git providers

You can configure Git OAuth via environment variables:

```sh
CODER_GITAUTH_0_TYPE=github|gitlab|azure-devops|bitbucket
CODER_GITAUTH_0_CLIENT_ID=xxxxxx
CODER_GITAUTH_0_CLIENT_SECRET=xxxxxxx

For your OAuth application, set the callback URL to:
/gitauth/<type>/callback
```