docs: clarify run.yaml files are starting points for customization (#2746)

# What does this PR do?
This PR improves documentation clarity around run.yaml file usage. It
adds comprehensive guidance to help users understand that generated
run.yaml files are templates meant to be customized for production use,
not used as-is.

## Changes
- Add new documentation section on customizing run.yaml files
- Clarify that generated run.yaml files are templates, not production
configs
- Add guidance on customization best practices and common scenarios  
- Update existing documentation to reference customization guide
- Improve clarity around run.yaml file usage for better user experience

## Test Plan
- Verified new documentation file exists at correct location
- Confirmed documentation is properly integrated into the toctree
structure
- Checked all internal links use correct paths and reference existing
files
- Validated references are added to relevant existing documentation
files
- Documentation build testing will be handled by CI environment

docs/source/distributions/building_distro.md

@@ -145,6 +145,10 @@ $ llama stack build --template starter
 ...
 You can now edit ~/.llama/distributions/llamastack-starter/starter-run.yaml and run `llama stack run ~/.llama/distributions/llamastack-starter/starter-run.yaml`
 ```
+
+```{tip}
+The generated `run.yaml` file is a starting point for your configuration. For comprehensive guidance on customizing it for your specific needs, infrastructure, and deployment scenarios, see [Customizing Your run.yaml Configuration](customizing_run_yaml.md).
+```
 :::
 :::{tab-item} Building from Scratch