Add semgrep sast analyzer docs and template

What does this MR do?

• Add docs for the new semgrep analyzer
• Update the SAST CI Template to include a job to run the new semgrep analyzer

Related Issue

#299585 (closed)

Manual QA

!53815 (comment 506444541)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content