Add a note about doppelganger protection interoperability to the docs (#2607)
Add a note about doppelganger protection interoperability to the docs (#2607)

## Issue Addressed

N/A

## Proposed Changes

Add a note to the Doppelganger Protection docs about how it is not interoperable until an endpoint facilitating it is standardized (https://github.com/ethereum/beacon-APIs/pull/131).

## Additional Info

N/A


Co-authored-by: realbigsean <73195634d109c8d8533133f62102413ab6837100@gmail.com>

sigp/lighthouse,sigp/lighthouse,sigp/lighthouse,sigp/lighthouse