Add instructions how to add a manuall PAT secret

https://github.com/eclipse-che/che-docs/pull/2383 requires editing several files:

Minimal steps:

✔ 1. Edit
https://www.eclipse.org/che/docs/next/end-user-guide/using-credentials-and-configurations-in-workspaces/.
✔ 2. Create a new page to parent
https://www.eclipse.org/che/docs/next/end-user-guide/using-a-git-credentials-store/
and
http://0.0.0.0:4000/docs/next/end-user-guide/configuring-a-git-provider-access-token/.
3. Reference the new parent in https://www.eclipse.org/che/docs/next/end-user-guide/using-credentials-and-configurations-in-workspaces/.
✔ 4. Edit
https://www.eclipse.org/che/docs/next/end-user-guide/authenticating-yourself-to-a-git-server-from-a-workspace/.

5. Open a PR with the edited files against the branch of https://github.com/eclipse-che/che-docs/pull/2383.