Document the steps required to rebuild the dashboard cache

### Description

There is currently no documentation on how to rebuild the cache for the security dashboard. When the dashboard needs to be refreshed, there's no runbook to follow, we don't know the exact steps, who has access, or what the expected outcome looks like.

### Goals

- Document the step-by-step process to rebuild or refresh the cache
- Note any required access, credentials, or permissions
- Add the runbook somewhere the team can find it

### Non-goals

- Changing how the cache works
- Automating the rebuild (that can come later once we understand the process)