Add docs for Group (SaaS) User Caps

What does this MR do and why?

This adds the initial documentation for Group (SaaS) User Caps, and the new Pending Members page, introduced in #333064 (closed).

This resolves #334676 (closed) and #343340 (closed)

Screenshots or screen recordings

Not applicable. To see the docs with the changes of this MR, click on "view latest app" link below.

http://main-ee-77567.178.62.207.141.nip.io/ee/user/group/index.html

How to set up and validate locally

Not applicable. To see the docs with the changes of this MR, click on "view latest app" link below.

http://main-ee-77567.178.62.207.141.nip.io/ee/user/group/index.html

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.