fix(docs): update core docs

[ixje]

Yesterday somebody pointed out that the core docs where not completely up to date either. I updated all of them except for the tx.md one (more on this later). The goal was to make all samples and text valid again. I updated wallet addresses and transaction id, hashes and such to point valid Neo3 values

As for tx.md I'm not sure what would be valuable content. The majority of it now describes the different transactions we had in Neo2. Stuff like this are no longer a thing

Neon.create.claimTx(...args);

The only functions we could describe are addWitness, addSigner and maybe (de)serialize. addAttribute exists but is obsolete NEO2 stuff.

[ixje]

Took a stab at updating tx.md. Tx building now happens through the TransactionBuilder, which is actually part of neon-api not neon-core so kind of falls outside of this documentation