Update docs for gitlab-advanced-sast-cpp: Move ruleset configuration to SAST docs from developer docs in the analyzer repo

What does this MR do and why?

This MR moves the ruleset configuration section from the developer documentation in the analyzer repository to the SAST documentation. As gitlab-advanced-sast-cpp approaches GA, this content is more relevant for users than for developers.

(Another analyzer MR Remove the ruleset config section from the deve... (gitlab-org/security-products/analyzers/clangsa!49) • Hua Yan • 18.6 will handle the removal of the ruleset configuration section from the analyzer repository.)

References

clangsa-sast: update documentation (#559112) • Hua Yan • 18.6

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