Create GLAS diff-based scanning user facing documentation

Summary

Create GLAS diff-based scanning user docs

Please read gitlab-com/content-sites/handbook!13575 (merged) for full details of how this feature is intended to work.

References

Main diff-based scanning epic: Faster Advanced SAST: Diff-based scanning in MRs (&16790 - closed)

MVC implementation epic: MVC: Enable Diff-Based Scanning in MRs for Fast... (&17758 - closed)

Implementation Plan

  1. Explain how GLAS diff-based scans work
    1. Scans only files that were added or updated, excludes removed files
    2. Scans dependent neighborhood files (see this document for detailed explanation)
  2. Provide steps to configure the scan
    1. using the SAST_PARTIAL_SCAN CI variable in the gitlab-advanced-sast job
    2. Ensure that the AST_ENABLE_MR_PIPELINES CI variable is enabled as well. See context
  3. Explain that it does not report fixed vulns as there's a possibility of false negatives. See context
  4. If users need to trigger a full scan, they must set SAST_PARTIAL_SCAN=false. However, this cannot be done using pipeline variables, as this feature requires a MR pipeline, which does not support manual pipeline variables.
Edited by Shao Ming Tan