Add RPC error codes documentation (fixes bitcoin-dot-org/Bitcoin.org#1837)

[zp6]

Summary

This PR adds comprehensive RPC error codes documentation to address bitcoin-dot-org/Bitcoin.org#1837.

Changes

New file: reference/rpc_error_codes.rst
A complete reference page documenting all Bitcoin Core RPC error codes, organized by category:

• Standard JSON-RPC Errors (-32700 to -32603): Parse errors, invalid requests, method not found, invalid params, internal errors
• General Application Errors (-1 to -32): Misc errors, type errors, invalid address/key, out of memory, invalid parameter, database errors, deserialization errors, verify errors, warmup, deprecated methods
• P2P Client Errors (-9 to -34): Not connected, initial download, node management, invalid IP/subnet, P2P disabled, capacity reached
• Chain Errors (-33): Mempool disabled
• Wallet Errors (-4 to -36): Insufficient funds, keypool exhausted, encryption errors, wallet not found, wallet already loaded/exists
• Block Validation Reject Reasons: Descriptive messages like bad-fork-prior-to-checkpoint (code 67), missing-inputs, insufficient-fee, etc.
• Regtest Troubleshooting: Specific guidance for the bad-fork-prior-to-checkpoint error from the issue, plus other common regtest issues (removed generate command, wallet setup, insufficient funds)

Each error code includes:

• Numeric code
• Constant name
• Description
• Resolution/solution

Modified: reference/index.rst
Added rpc_error_codes to the toctree.

Modified: examples/testing.rst
Added a "Troubleshooting Regtest Errors" subsection with the example from the issue (bad-fork-prior-to-checkpoint (code 67)) and a link to the full error codes reference.

Error Code Source

Error codes are sourced from src/rpc/protocol.h in the Bitcoin Core repository.

Closes bitcoin-dot-org/Bitcoin.org#1837