Document what the public key is for each keytype

[onbjerg]

We should document what goes into publicKey in the Key struct depending on the keytype. For example, for Secp256k1 it is just the corresponding address of the private key, i.e. not actually the public key but a truncated hash of it.

[atarpara]

Key Type	Encoding Format	Description
secp256k1	abi.encode(address)	Stores only the Ethereum address derived from publicKey(truncated hash).
webAuth	abi.encode(x, y)	Stores both x and y coordinates of the public key on the elliptic curve.
secp256r1(P256)	abi.encode(x, y)	Stores both x and y coordinates for the secp256r1 curve.