Improve the FAQ explaining the mismatching validator client key share index

[boulder225]

🎯 Problem to be solved

The issue revolves around an invalid setup or deployment, where keyshares are not tied to the enr private key. There appears to be a mix-up during deployment, leading to a mismatching validator client key share index. Specifically, Bob's key share is submitted to Alice's charon node, and this has confused, as evidenced by the error message that has come up multiple times.

node5_1           | 10:51:24.001 ERRO vapi       Validator api 5xx response: mismatching validator client key share index, Mth key share submitted to Nth charon peer {"status_code": 500, "message": "Internal server error", "duration": "93.309µs", "key_share_index": 2, "charon_peer_index": 5, "vapi_endpoint": "get_validators"}
node5_1           |     core/validatorapi/validatorapi.go:102 .func3
node5_1           |     core/validatorapi/validatorapi.go:986 .ValidatorsByPubKey
node5_1           |     core/validatorapi/router.go:1166 .getValidatorsByID
node5_1           |     core/validatorapi/router.go:290 .func1
node5_1           |     core/validatorapi/router.go:268 .func1
node5_1           |     app/app.go:788 .func1

🛠️ Proposed solution

• Create a Guide with Examples: Utilize the Alice and Bob example to craft a clear and concise explanation of the problem. This will aid in understanding and potentially prevent future occurrences.
• Add to FAQ: Include the explanation and solution in the FAQ to make it readily accessible to anyone encountering this issue in the future.

Thread: https://obollabs.slack.com/archives/C05G36J6Z7Y/p1692721291936209