[Flang][driver] Update the docs

Please note that the updated documentation reflects the design that we are
working towards, but haven't implemented competely yet. We took this approach
in order to provide a more holisitic and complete overview of the design.

In particular, this document assumes that Flang's frontend and compiler
driver can already generate code. This is still work-in-progress and is
being developed on the `fir-dev` branch in the F18 repository [1].

[1] https://github.com/flang-compiler/f18-llvm-project/tree/fir-dev

Differential Revision: https://reviews.llvm.org/D111573