Document MAVEN_CLI_OPTS and other environment variables

What does this MR do?

It adds documentation for MAVEN_CLI_OPTS, a customization environment variable that's asked by several of our clients. It seems important to mention it in the documentation. It also document other environment variables for the find-sec-bugs analyzer.

What are the relevant issue numbers?

Refs https://gitlab.com/gitlab-org/gitlab-ee/issues/8689

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added
• [ ] Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• [ ] Tests added for this feature/bug
• [ ] Job definition updated, if necessary
  • [ ] Auto-DevOps template
  • [ ] Job definition example
  • [ ] CI Templates
• Conforms to the code review guidelines
• [ ] Conforms to the Go guidelines
• Security reports checked/validated by reviewer