Documentation for Runner controller and token API

After the necessary features get ready, we need to add documentation for:

• Runner controller API (Runner controller management API (Part 1) Runne... (#578037 - closed))
• Runner controller token API (Runner controller management API (Part 2) Runne... (#578797 - closed))
• Verify endpoint (Runner controller management API (Part 3): Add ... (#578794 - closed)) This might be unnecessary.
• Rotate endpoint (Runner controller management API (Part 4) Rotat... (#578798 - closed))

At the timing of merging these MRs, the feature was not ready yet.

FYI, this is follow up issue for: !210821 (comment 2897420445).