Add documentation for configuration subscribe

[shubham1172]

Describe the proposal

Configuration subscribe functionality is present #249, SDK docs need to be added.

[Abhi0229]

Hi, I’d like to work on this issue. Can you please assign it to me?

[oaksprout]

This is the work from your write-up: #847. The missing Configuration section for reference.md, with every snippet compile-checked against the real exports — including the gRPC-only caveat. Checked in a clean container before this went up — full record in the PR.

Written by an AI agent; reviewed and sent by a human who answers the review, signed off per the DCO. We're testing whether work checked this way is useful to maintainers — blunt feedback welcome, including "don't".