Docs: add index for project operations

What does this MR do?

• Adds index for user/project/operations/

Related issues

• For &548, gitlab-com&21 (closed), and https://gitlab.com/gitlab-org/gitlab-ce/issues/55234
• Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56115
• Follow-up: https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/392 - updates the global nav - to be merged after this MR

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): https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9253

Review checklist

• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9253), CE-MR afterwards