[dynamic control] Update README on instructions for using the project as an extension

[jackshirazi]

Description:

Update README on instructions for using the project as an extension. I've included the declarative config instructions although they aren't valid until #2881 is merged.

Existing Issue(s):

part of #2868

Testing:

Manually tested

Documentation:

That's what this is

Outstanding items:

see #2868

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds comprehensive user-facing documentation to the dynamic-control module's README, describing the Telemetry Policy concept, build/usage instructions, configuration options, and an end-to-end example.

Changes:

• Adds "Using this project" section explaining the Telemetry Policy flow (Message → Provider → Policy → Aggregator → Implementer).
• Documents building the extension, using it via auto-config or declarative config, and lists supported config values and policies.
• Provides a Unix quick-start example and a worked configuration example.