Add developer documentation for integration of Indexed staking

Document here <https://hackmd.io/@d1ll0n/SkJPU8Lsd>

This document should be added in a new "Integration" section of the docs.

Needed improvements:
- Better organization of the document
- Clear explanations of the return values for queries
- Explanation of prerequisites for the interactions & effects
- Details on how to get updated reward values after initial query