Add a `TransactionResultCode` page with an explanation

[jeesunikim]

rpc.sendTransaction returns result (errorResult.result).

Some of the responses can be unclear. It would be great to have what each status means. TransactionResult types can be found here

[briwylde08]

Need a section like this: https://developers.stellar.org/docs/data/apis/horizon/api-reference/errors/result-codes/transactions for RPC in our docs.