[docs][fidl] Document fidl compiler interface

Change-Id: Id682b7cfb76223744f2bc8a5a8a3136ed7fff3b8

[fidl][docs] Link to implementation docs from overview docs

Change-Id: If7df03ed76f151cb0b0b9f16d33ded1fa7006693

[fidl][docs] Refer to fidl as fidl, not fidl2

Change-Id: Id67dc99b0819388586bd43b0794474dfb908f0d5

[docs][fidl] Remove some cruft from fidl docs

While the implementation is in progress, the design more-or-less is
not.

Change-Id: If8cc17c99db63545b799d0d11a8337a018288bf1

[docs][fidl] Create FIDL markdown documents.

These were converted from google docs automatically, then fixed up
manually.

Note these may not be up to date. kulakowski@ will work on updating them
when he gets back from vacation.

TO-321

Change-Id: If974bcb00a46bfe605a1005287b7b31df717f427