Documentation for SAST

This issue tracks SAST (https://gitlab.com/gitlab-org/gitlab-ee/issues/3723) documentation.

We need to add documentation in two places:

• Auto DevOps documentation should include Auto SAST as part of the process
• Merge Request documentation should include SAST results in MR widget, similar to Code Quality ~"EE Ultimate"

Note: for Auto DevOps documentation, we should include in CE documentation.

See https://gitlab.com/gitlab-org/gitlab-ee/issues/3723 and https://gitlab.com/gitlab-org/gitlab-ee/issues/3775 for additional details.