Verify mnemonic using existing-mnemonic Documentation

[valefar-on-discord]

There is a scenario where, after an extended period of time, a user may want to verify that the mnemonic they have is indeed the initial mnemonic that was used during their validator keystore generation.

The best approach to do this as of now is utilizing the existing-mnemonic flow and correctly regenerating a validator keystore.

We should add a section to the documentation talking about how this flow can validate mnemonic possession and provide instructions on how to execute the command and how to verify the pubkey.