Add better documentation on CallbackBlocks

[aantn]

CallbackBlocks are part of Robusta's API. They're used to write playbooks where the user clickes a button (e.g. in Slack) and this triggers another playbook which runs only when the user clicked.

This powers, for example, the playbook that lets you increase the HPA max replicas. (See docs)

Today CallbackBlocks aren't documented well in the docs on writing playbook actions

We should document them there in a new page in that section of the docs. The page will be all about callbacks, how they work, and how to write a playbook that uses them.

For reference, you can search for CallbackBlock in the playbooks/ directory and read the existing playbooks to see how it works.