Docs: Accept automatic upgrades within a specific channel

[perdasilva]

Write user documentation describing how to keep automatic upgrades to a specific channel. If documentation already exists, review it and ensure it is still accurate and up-to-date.

A/C:

• Describe what a channel is
• Instructions to specify a channel for auto-upgrades
• Call out that you can also use version ranges to further restrict the set (of allowable versions)
• Call out auto-upgrades from multiple channels

Reach out if you have any questions please reach out on Slack

Current documentation is posted up at https://operator-framework.github.io/operator-controller/
New docs should be placed in docs/drafts