docs: add Git remotes authentication guide

[emanuelpalaszewski]

Why

Users need a concise, GCM-specific explanation of how HTTPS and SSH remotes differ and when GCM participates in authentication.

What changed

• Added a guide covering HTTPS authentication through GCM, credential storage, token safety, SSH setup, and switching remote URLs.
• Linked the guide from the documentation index.

Validation

• git diff --check

[dscho]

I do not find this documentation particularly helpful, nor do I agree that GCM is the right spot for this documentation: This is about Git remotes, therefore it should be documented in Git. But then, the documentation is misleading in the first place because GCM kicks in as a credential helper, not only for remotes, but even for clones (where there is no remote yet) or for plain-URL fetches ("git fetch https://github.com/git-ecosystem-git-credential-manager main").

It doesn't help that this is the usual AI-generated verbose word soup.

tl;dr I would not merge this, nor any iteration thereof.