Docs: add categories for each aspect of the project

This is not much, but once the directory layout is settled, it gets easier to choose where to put things.

Here I propose the following (but feel free to suggest any change!):

Hardware would offer an overview of the PCB, document the pinout, block diagrams, modifications...
Software would offer an overview of the SDK, Get Started instructions, document its API.
Gateware would document the FPGA: loading designs onto it, document the example wishbone interface, explain the toolchain for those new to FPGAs at all wanting to get started through the pico-ice board.

Also a comment removed.

tinyvision-ai-inc / pico-ice