Update FlatRedBall profiling documentation

[profexorgeek]

Recently our profiling process when FPS dips is to:

1. Break just before situation where dip happens
2. Start CPU profiling
3. Unbreak and run through slow-performing code
4. Break to stop profiling
5. View high-percentage CPU hot path to diagnose FPS dip

The existing profiling docs are pretty outdated and focus on profiling in code.

Visual Studio debug Window options are confusing. The action the user is taking is profiling the CPU. But if you try to start profiling the CPU from the Performance Profiler, it throws an error and says it's already profiling. This is because the actual profiling takes place in the Diagnostics Tools dialog.

FYI @andysterland the VS error message here is frustrating. It tells me another profiling session is active but gives me no indication of where to find it:

[vchelaru]

For reference, the profiling docs have moved to this location:

https://docs.flatredball.com/flatredball/tutorials/code-tutorials/tutorials-a-walkthrough-on-improving-performance

I'm in the process of updating these docs now.