clangsa-sast: update user facing documentation

Problem to solve

Clang static analyzer requires a compilation database(CDB), and users may not be familiar what a CDB is or how to produce one.

Proposal

Use the developer documentation as a starting point for topics and content.

We should include

• an explanation for what a "compilation database" is and why a C/C++ static analyzer might want one
• a list of common methods for generating a CDB for use with clangsa-sast
• example jobs
  • simple example that creates a CDB in a job and passes it as an artifact
  • use of cdb-rebase to relativize a CDB in before_script
  • copying header files referenced by the CDB into the analyzer image
  • using a GL cache for projects with CDBs that don't change often
    • the CDB of a project will only change when source/header files are added/removed and when flags change
• troubleshooting problems that originate with the compilation database

Finally,

• decide on where to put user docs, filter developer docs, and move
• update references to the documentation in the log messages of the clangsa analyzer, e.g. https://gitlab.com/gitlab-org/security-products/analyzers/clangsa/-/blob/ee11546f878b1a8c2bc62692265fcc1ac6d944f3/analyze.go#L28