Explicitly ention root hash is not stored anywhere

[yoshuawuyts]

In the Merkle Tree & Terminology sections we should emphasize the root hashes aren't persisted anywhere, but only the signatures of the hashes are persisted.

Also emphasize index for the signature is the index of the right-most node in the tree, divided by two. We should update our images to the following:

tree (stays same)

0: #0 = hash(data[0]) ─┐
                       #1 = hash(tree[0] + tree[1])
1: #2 = hash(data[1]) ─┘

2: #4 = hash(data[2])

signatures (added indexes)

0: #0 sig(#0)
1: #1 sig(#1)
2: #2 sig(#1 + #4)