Update docs to reflect changes to Indexer query fee rebating

[alex-pakalniskis]

Hey The Graph Academy,

There are some protocol changes to Indexer query fee rebating. The Cobb-Douglas function has been replaced with an exponential rebate function. Please see https://thegraph.com/docs/en/network/indexing/

I'm opening this issue to encourage a documentation update to The Graph Academy documentation to reflect these changes.

Please let me know if you'd like some support.

Best,
Alex