The revive compiler documentation (#424)

This PR adds comprehensive project documentation in the form of an
mdBook.

---------

Signed-off-by: xermicus <cyrill@parity.io>
Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>
Co-authored-by: LJ <81748770+elle-j@users.noreply.github.com>
Co-authored-by: PG Herveou <pgherveou@gmail.com>

.ignore

@@ -1,2 +1,3 @@
 # This is no longer in .gitignore; we add it here (honored by tools like rg and fd).
 llvm/
+docs/