docs: document the process for getting a maintenance release

[SamBarker]

Summary

• Documents the project's roll-forward release policy and explains how community members (including committers) can get a maintenance release cut for an older version
• Clarifies that base image respins are not a reason to cut a maintenance release
• Describes the lazy consensus process committers follow before cutting a release

Test plan

• Preview rendered at http://127.0.0.1:4000/release-schedule/

[robobario]

LGTM