[doc] Add docs for FFI performance trouble shooting and improvements

[dcharkes]

ways to assess performance:

• dart developer timeline events
• Linux perf utility for Dart standalone (especially single-threaded/single-isolate performance) to see time spent in both Dart and native code

some simple tips and tricks for making some FFI patterns faster:

• avoid copying data, keep it in native memory if you can
• if you have many small calls, use leaf calls
• if you want to avoid copying data from Dart to native, and the call is short, use a leaf call and the .address.
• if you use a helper isolate, use send-and-exit (not really FFI specific, but more common to use with FFI).

[orestesgaolin]

I think talk by @RobertOdrowaz is a nice example of benchmarking tasks using ffi and other approaches
https://www.youtube.com/watch?v=eJR5C0VRCjU