Update SAST Analyzer development guide

What does this MR do and why?

Adds a section to explain the criteria used by the SAST feature to match the correct analyzer to a project.

Relevant Slack discussion (team members only)

Screenshots or screen recordings

How to set up and validate locally

Setup GitLab docs locally and preview at ee/development/sec/analyzer_development_guide.html#execution-criteria

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.