Add profiling documentation

[mikegre-google]

re: issues/8743

This issue has a request for adding documentation on the start_trace and stop_trace API, but we currently don't have any documentation around profiling. Who can I work with to get some profiling documentation written? Thanks!

[ysiraichi]

@zpcore @tengyifei @bhavya01

[tengyifei]

I think @lsy323 added this

[yaoshiang]

I filed a related issue. The current "overview" is too detailed and long for an overview. #8774.

But the overview happens to have a section on profiling. I'll discuss internally on whether it makes sense to break it out - reducing the complexity of the "overview" while also creating a profiling guide.