Improve Rule ID mapping documentation for sast-rules
Problem to solve
The whole rule mapping strategy used in https://gitlab.com/gitlab-org/security-products/sast-rules is not explained. We should add more detail about how the rule mapping strategy currently works.
Proposal
Create a section in https://gitlab.com/gitlab-org/security-products/sast-rules/-/blob/main/README.md?ref_type=heads that explains the rule id mappings and the ratinoale(s) behind it.