feat(docs): document how to fill from an EELS dev branch

The "Filling Tests for Features under Development" needs updating to include docs on how to fill tests from an EELS development branch, i.e., how to specify the EELS definition of a development/custom fork in `eels_resolutions.json`.

This section is still very geth-oriented:
https://ethereum.github.io/execution-spec-tests/main/filling_tests/filling_tests_dev_fork/