Link to git-credential-oauth

What does this MR do and why?

Link to credential helper git-credential-oauth from the two-factor authentication page. The page already mentions another credential helper Git Credential Manager. While they have similar features, it's worth mentioning both, because each has advantages. In particular, git-credential-oauth is included in several Linux distributions.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.