Update SAML Group Sync documentation and add link from UI

What does this MR do?

Related to #331670 (closed)

Attempts to clarify how SAML group sync works in terms of adding/removing members. This MR adds a note to the UI and link to the docs. This MR also adds a diagram to the docs to help explain how SAML group sync works.

Screenshots or Screencasts (strongly suggested)

Before	After

How to setup and validate locally (strongly suggested)

1. Setup SAML in your GDK instance: https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/saml.md
2. Create a group
3. Navigate to Settings -> SAML SSO and setup SAML
4. Navigate to Settings -> SAML Group Links

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• [-] I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.