Update user pays model docs to include info about new resource usage

[reynaldichernando]

We recently improved our credits/resource usage with our metering system. This allows us to see actual usages of our puterjs APIs call

It might be a good idea to update our docs to add this info, so we have more clarity on what the "user covers their own cost" statement.

Which just means, every user has their quota of resources (as seen from the image), and everytime you use an app with the puterjs features, you will just be consuming your own resources.

[reynaldichernando]

related to this, but non goal for this issue is specifying how much resource each action takes, as i believe this is still internal and ongoing process to be shared

it might have its own separate place, since we have so many APIs