Add SAST_SCANNER_CLI_OPTS CI var information in the SAST Documentation
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Add `SAST_SCANNER_CLI_OPTS` CI variable information in the SAST Documentation. This variable accepts hand-picked flags to forward to the underlying scanner's CLI opts. More information about this is discussed [here](https://gitlab.com/gitlab-org/gitlab/-/issues/337530#note_1018563265).
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