Gitlab integration docs

[ashearin]

Description

Supercedes https://github.com/DependencyTrack/hyades/pull/1809 and https://github.com/DependencyTrack/hyades/pull/1820

Related to https://github.com/DependencyTrack/hyades-apiserver/pull/1325 and https://github.com/DependencyTrack/hyades-frontend/pull/318

Addressed Issue

Additional Details

Checklist

• [ ] I have read and understand the contributing guidelines
• [ ] This PR fixes a defect, and I have provided tests to verify that the fix is effective
• [ ] This PR implements an enhancement, and I have provided tests to verify that it works as intended
• [ ] This PR introduces changes to the database model, and I have updated the migration changelog accordingly
• [ ] This PR introduces new or alters existing behavior, and I have updated the documentation accordingly

[owasp-dt-bot]

:tada: Snyk checks have passed. No issues have been found so far.

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