Adding a page on fee estimation

[odednaor]

Is your feature request related to a problem? Please describe.
The docs don't contain a description of how to calculate fees (should be done based on the starknet_estimateFee call), or how to map the fields returned by the call to the resource_bounds dictionary of v3 transactions.

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[stoobie]

@odednaor Did you already open a PR for this?