1. Update the documented NetBSD version.
2. Add the optional ZeroMQ package to align the guide with other *BSD
systems.
3. Update the Python version to meet the minimum requirement specified
in https://github.com/bitcoin/bitcoin/pull/30527.
4. Install `net/py-zmq` package to enable the `interface_zmq.py`
functional test.
5. Fix a formatting issue.
NetBSD doc has not seen any meaningful contribution since 2018.
This PR intends to update the docs so that one can successfully build on
the latest NetBSD release. It also adds dependency information and
instructions to build the GUI.