docs(fill): explain how to verify fixture refactors via hasher

[danceratopz]

Add something along the lines of the following which was taken from ethereum/execution-spec-tests#1170 (review)

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For future reference, here's how I verified fixtures locally:

Fill with your branch: with flag rm -rf fixtures-1170/; fill --fork Shanghai tests/shanghai/eip3860_initcode/test_initcode.py ----output=fixtures-1170
Fill with main: rm -rf fixtures-main/; fill --fork Shanghai tests/shanghai/eip3860_initcode/test_initcode.py ----output=fixtures-main
Run hasher on both output dirs:

uv run hasher fixtures-1170 > after.txt
uv run hasher fixtures-main > before.txt

Diff hasher output:

diff before.txt after.txt

[winsvega]

can there be one command. like in this test repo there is a CI running
-t GeneralStateTests --checkhash

and it verifies that all the generated tests have a corresponding filler hash in their header at the time of generation.
so if anyone commit outdated test which source filler is of a newer version it will yell an error