docs: document header validation process

At a high level, we have validation and verification.
* [Validation](https://github.com/celestiaorg/celestia-node/blob/main/header/header.go#L112) checks the integrity of all the fields in the deserialized header. Essentially comparing correspondence between RawHeader, Commit, and ValidatorSet fields.
* [Verification](https://github.com/celestiaorg/celestia-node/blob/main/header/verify.go#L14) involves header verification against each other via commitment signature verification of validator set