doc: Refactor coverage documentation

[prcr]

The main page with instructions for setting up coverage is getting too big, making it hard to follow through the complete sequence of instructions.

This pull request:

• Moves the instructions for more advanced scenarios to a dedicated page
• Tweaks a few references and admonitions so that they appear in the sequence that makes the most sense while users are configuring their setup

These improvements will give way to more detailed troubleshooting instructions for the most common coverage setup failure scenarios (see CY-6263).