Improve and consolidate credentials info

What does this Merge Request do and why?

Per this comment: !2442 (comment 850517786)

The credentials information should be better consolidated and easier to find.

This MR adds a dedicated 'Get the login credentials' section to the GDK commands page, which we can link to from other sections of the docs.

To accommodate this change, I had to clean up some of the other commands on that page and add them to their own sections.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.