[Docs] Add Python caveats under the development section
This adds a page named Caveats with a section on some of the things to
be aware of related to Python. It's a question we've seen more than once
pop up and I think it's good to have it documentation on the website.
Even though some of it might be useful to users, I still put it under
"development" because it requires some understanding of how LLDB is
Differential revision: https://reviews.llvm.org/D70252