Update Grades Docs

[ramanxg]

At ZotHacks, a project used our API for the grades data. However, even though it might have been easier to use the calculated version, they used the raw version and calculated it themselves. It's a little unclear what the /grades/calculated endpoint does, and we could help it by adding more details about it, or even moving it to the top before the /grades/raw.

TODO:

• [ ] move /grades/calculated above /grades/raw
• [x] Add to Exclude P/NP to be Exclude classes taken only P/NP.
• [ ] Specify /grades/calculated creates sum aggregates for the numbers.
• [ ] Repeat the list of parameters rather than saying it is elsewhere.