Code blocks on RPC sendTransaction page fail

[leighmcculloch]

This issue captures a couple issues with the examples on the sendTransaction RPC page:

1‍⃣ The Python SDK guide on the RPC sendTransaction page does not work as it is missing a step.

https://developers.stellar.org/docs/data/rpc/api-reference/methods/sendTransaction

It shows creating a contract invocation and then signing and submitting the transaction without calling prepare_transaction. The transaction is guaranteed to error in this case because it's not configured with fees or resources.

To make the transaction succeed, at the least you need to call the following prior to signing:

transaction = server.prepare_transaction(transaction)

2‍⃣ A second issue is that all the examples (Python, JS, Java) still wouldn't work out of the box, because they references a contract that doesn't exist. Instead of using a random increment contract, the example could be written to use the CDLZFC3SYJYDZT7K67VZ75HPJVIEUVNIXF47ZG2FB2RMQQVU2HHGCYSC contract, which is the native asset on the test network. The example could get the balance of a known predictable address such as the network root account.

The example would then create a much better developer experience, be ready to go without the developer having to navigate so many errors.

cc @stellar/platform-eng