Document git-credential-oauth as alternative to personal access tokens

Problem to solve

Credential helper git-credential-oauth is a user-friendly alternative to personal access tokens. The two-factor authentication page https://docs.gitlab.com/ee/user/profile/account/two_factor_authentication.html should mention it.

The page already mentions another credential helper Git Credential Manager. While they are similar, it's worth mentioning both, because each has advantages. In particular, git-credential-oauth is included in several Linux distributions.