core: add guidance level to performance audits

[adamraine]

@alexnj and I discussed changing our "difficulty" level to a "guidance" level since difficulty can be very page dependent, but we know how much guidance our audit is going to provide. These are the guidance level descriptions I used for this PR:

• 3: We give you specifics on what to fix and we tell you how to fix it
• 2: We give you specifics on what to fix but you will need to design a solution yourself
• 1: No specifics just a general overview, design a solution yourself

In the future, we could consider adding a 0 level for things like manual audits or even user-timings, but I don't see it being useful right now. These guidance levels could also be applied to non-performance audits.

All of these are up for discussion, this was my initial pass. We discussed, these values are now the best consensus so far.

[connorjclark]

Should we take some group votes on difficulty again? remember this?

[adamraine]

Should we take some group votes on difficulty again? remember this?

I'm hoping a switch to guidance would make things a little less subjective. I'm fine with making a spreadsheet to organize this and facilitate discussion, but I don't want to compute an "average" guidance level based on our individual rankings.

Edit: added a guidance level sheet to facilitate discussion