Add documentation about Neo X node extensions

[AnnaShaleva]

Rationale

We have a set of differences from the default Geth node behaviour introduced by either NeoXBurn fork (including affected RPC API handlers) or by other node modifications. Currently all these divergence from the base node behaviour are not documented. In future there will be more of them

Implementation

I propose to add ./docs/node_extensions.md file where we'll keep all the user-facing extensions properly documented. We may start with NeoXBurn behaviour changes and #263. When OnPersist system call may be added since it may affect in-block transactions processing.

[AnnaShaleva]

Shanghai withdrawals should be mentioned since it's an essential part of Shanghai and dBFT does not allow non-empty withdrawals.

[AnnaShaleva]

A part of Q1, but won't be done in 0.3.0.