Improve development sandbox documentation

_Generated by an AI assistant on behalf of @rpbritton._

### Problem statement

Raised as an action item in the Charm QA Triage Retrospective (9 Sept 2026): several attendees reported that environment setup and troubleshooting for the development sandbox took too much time. Existing docs don't cover cases such as CMR testing or re-using an existing controller for environments like testing on top of a local CK8s.

### Enhancement Proposal

Expand the development sandbox documentation to cover the gaps identified above, so new contributors can set up and troubleshoot the sandbox without needing to reverse-engineer command lines via trial and error (or an AI assistant).

### What needs to get done?

- Document CMR (cross-model relation) testing setup in the sandbox.
- Document how to reuse an existing controller for environments (e.g. testing on top of a local CK8s) instead of always bootstrapping fresh.
- Review and fill other gaps found during onboarding/troubleshooting.

Start by locating the existing development sandbox documentation and the current setup or troubleshooting entry points. Document CMR testing, reusing an existing controller for local CK8s environments, and other onboarding gaps found during review; done means contributors can follow the documented workflows without reverse-engineering commands.

Written by the indexing model from the issue text.