[docs] Make global constants more visible

[critesjosh]

Make the constants listed here more visible to developers. Feedback from a dev

I discovered that it is not currently possible to retrieve more than 16 notes at a time from the PXE due to existing limitations. This limitation is not well-documented or obvious within the current Aztec documentation

The global protocol constants could be imported to a docs page relatively easily using the #import_code macro.