clean up install instructions

[securelyfitz]

recording notes here that got me up and running and should eventually be reflected in instructions

• amaranth install instructions located at https://amaranth-lang.org/docs/amaranth/latest/install.html
• amaranth install instructions have LOTS of choices that might not be clear; I did sudo apt-get install python3-pip gtkwave and pip3 install --user --upgrade 'amaranth[builtin-yosys]'
• amaranth-boards is not installed properly nor instructions included, i did pip install git+https://github.com/amaranth-lang/amaranth-boards.git found in discord
• i had to also install the nextpnr toolchain - i installed the fomu toolchain , but perhaps the yosishq toolchain bundle would be a better choice?

After this, I've got the examples building and flashing without error.