Docs: add index for project operations

What does this MR do?

• Ports https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24565 to EE

Related issues

• For &548, gitlab-com&21 (closed), and gitlab-com&21 (closed)
• Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56115
• Follow-up: https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/392 - merge after the CE MR gets merged

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE):

Review checklist

• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24565) afterwards