[BoundsSafety] Initial documentation for -fbounds-safety (#70749)

The document is mostly the exact copy of RFC: Enforcing Bounds Safety in
C, except some clarifications made over the PR review:

https://discourse.llvm.org/t/rfc-enforcing-bounds-safety-in-c-fbounds-safety/70854

Further changes and clarifications for the programming model will be
done as separate patches to make it easier to track history of changes.

Author: rapidsna