Document coder_git_auth deprecation

[evilhamsterman]

In #163 coder_git_auth was marked as deprecated. It does show a message when used but it would be helpful to provide that same information in the documentation. Right now the documentation doesn't say anything about the deprecated state of coder_git_auth or mentions you should use coder_external_auth instead.

In fact because of the fact that the coder_external_auth mentions multiple services but not anything like git or GitHub (gh is mentioned but unless you know it you may not make the connection) people may be lead to believe they should use coder_git_auth for git authentication and coder_external_auth for authentication with other services. This may lead people to use one and not realize the deprecation until they first use it in a template. Unless like me they go looking for what the difference is and stumble on the PR.