Give formal index a proper title.
Use `Pass::formatted_help()` to assign the group, but still return `false` because the help text still comes from `Pass::help()`.
Tidy up some of the affected files' includes to make use of the shared `yosys.h` includes.
Removes group parsing from command ref domain, instead relying on a 'groups' object in the cmds.json file.
`docs/source/cmd` is no longer ignored or cleaned.
