Document Jenkin slave management.

This is correct-but-incomplete as I understand it: nodepool is not yet covered.

Change-Id: Ib45cdb94f876e51bd7c01aecad70f470923d9b52
Phase 3 infra bootstrap docs: gerrit.

These are still rough, but I'd appreciate an expert eye cast over
them.

Change-Id: If46abecd73456f143544f4507bf98385339a3f2e
Merge "Document Jenkin slave management."
Merge "Phase 3 infra bootstrap docs: gerrit."