Adds documentation for restricted project service account in deployment jobs

What does this MR do?

Adds documentation for restricted project service account in deployment jobs (which helps RBAC controls)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/51717

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated
• [-] Tests added for this feature/bug
• [-] Conforms to the code review guidelines
• [-] Conforms to the merge request performance guidelines
• [-] Conforms to the style guides
• [-] Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.