[docs] Improvements to the FAQ.

[ChrisCummins]

This restructures the HTML documentation's FAQ section and adds new questions.

Some tips for common debugging:

• Why does my environment’s step() function return “done”?
• How do I debug crashes or errors?

and for common development use-cases:

• I want to modify a CompilerGym environment, where do I start?
• I want to add a new CompilerGym environment, where do I start?

This replaces the previous question about adding a new observation / reward signal that was specific to the LLVM environment.

The goal is to provide some tips for common debugging scenarios, like #585 and #582.