[FIRRTL] Documentation and cleanup for the FIRRTL backend

[ayakayorihiro]

This PR contains some small follow-ons to the FIRRTL backend:

• We now use firtool instead of the deprecated FIRRTL compiler in fud2 (thanks @ekiwi!). Note that I changed fud2's configurations for FIRRTL.
• We also use the external-to-ref pass instead of a sed-based hack for converting @externals to refs.
• I wrote some documentation on running the FIRRTL backend!

Other than some relatively minor things (like playing around with firtool arguments), I'm hoping this will tie a small bow around the FIRRTL backend work until we want to pick it back again for ESSENT simulation :)