Clarify feature tiering in SAST documentation

Problem to solve

A non-ultimate GitLab user noted that the documentation for SAST doesn't properly differentiate which features are available only for ultimate users. In this case, the user tried to navigate to the security dashboard to view their SAST results only to learn that they do not have access.

Customer feedback is from https://www.reddit.com/r/gitlab/comments/p6p29v/how_to_see_gitlabci_sast_report/

Proposal

TBD