Write developer documentation for core hardware code

[alexdewar]

The core hardware module is already somewhat complicated, but since #336 it has become more so (albeit a bit cleaner!).

As this is potentially a barrier to contributing, the developer documentation for the hardware code should be improved. This could be done either with longer module docstrings or with separate markdown files (or both).

It should include:

• Explaining concepts like device base type and device type
• Explaining how to make a new plugin (including making new base types and regular device types)
• Better API documentation for the current base types
• Descriptions of the pubsub messages that are used
• A better module description for data_file_writer.py (possibly, depending on whether we're going to refactor it or not)