docs(fill): document how test a new eels fork/eip in eest

[danceratopz]

Document how to add new tests for a new EELS fork by modifying:

• eels_resolutions.json, and,
• src/ethereum_test_forks/forks/forks.py.

and running uv run eest make test.

I realised we don't have these docs here https://discord.com/channels/595666850260713488/1329478953705934890/1329482261044920453