[Docs] Add developer documentation for using/modifying the chrome service

[joshuarrrr]

As a plugin or core dashboards developer, I'd like to have a README for the chrome service to be able to quickly and consistently integrate plugins with the global application chrome.

Possible questions to answer:

1. What is the chrome service responsible for?
2. What are the common integration points plugin authors should be aware of (commonly used getters/setters)?
3. Are there any key limitations, guardrails, or bounds to be aware of (min/max nav links, breadcrumb depth, etc.)

While a README is probably the quickest and easiest place to start, we could also add a new example that demonstrates usage.