docs: allow multiple Coder deployments to use single GitHub OAuth app

[matifali]

If you have multiple Coder deployments and want to log in with GitHub OAuth.
Coder passes the redirect-uri in the form,

coder/coderd/gitauth/config.go

Line 245 in f6f61df

authRedirect, err := accessURL.Parse(fmt.Sprintf("/gitauth/%s/callback", entry.ID))

And it matches the now updated callback URL as per their docs

The redirect_uri parameter is optional. If left out, GitHub will redirect users to the callback URL configured in the OAuth app settings. If provided, the redirect URL's host (excluding sub-domains) and port must exactly match the callback URL. The redirect URL's path must reference a subdirectory of the callback URL.

CALLBACK: http://example.com/path

GOOD: http://example.com/path
GOOD: http://example.com/path/subdir/other
GOOD: http://oauth.example.com/path
GOOD: http://oauth.example.com/path/subdir/other
BAD:  http://example.com/bar
BAD:  http://example.com/
BAD:  http://example.com:8080/path
BAD:  http://oauth.example.com:8080/path
BAD:  http://example.org

Originally from #8714
I have tested this with my own deployment and PR deployments.