[Guide] Sign transactions offline

[dgarcia360]

Create a guide explaining how to define and sign a transaction without internet connection using the CLI (and SDK if possible). Signing transactions offline is useful to sign transactions from cold wallets with no internet access. By copying just the signed tx payload, it's possible to announce the transaction from another computer that can reach the internet without compromising the account private key.

Possible structure:

1. symbol-cli transaction transfer
2. Do not announce the transaction, copy the signed payload.
3. Move to a computer with internet connection
4. Announce the transaction (with curl or symbol-cli transaction uri) https://nemtech.github.io/concepts/transaction.html#announcing-a-transaction