g-ir-compiler: remove references to --code, remove --no-init options

The "--code" option was removed years ago in ac81f3e8c5f1c380d16677232d67466e739da283
so remove references to it from README and g-ir-compiler(1)

Remove the "--no-init" option from g-ir-compiler and g-ir-compiler(1)
as it was documented to "can only be used if --code is also specified",
so no reason to keep it around.
build: actually require libffi >= 3.0.0

as documented in README...
README: fix the Tools section a bit
[WIP] docs: improve layout
!!! for testing only !!!