[clang] [BoundsSafety][Doc] Add BoundsSafetyAdoptionGuide.rst (PR #120674)

[Yeoul Na via cfe-commits]

rapidsna wrote:

> Given the number of small changes that need to be made, I think it's worth mentioning that fix-its exist and how their fixes can be applied, since that's not always obvious when building in the terminal. Perhaps we also want to mention that we'd appreciate reports of snippets that compile without errors but are optimised to unconditional traps?

Sounds good. Let's add those in a follow up since we also need to add debugger support.

https://github.com/llvm/llvm-project/pull/120674