docs: clarify grafana additional_dashboards requires a github/http locator

[barnabasbusa]

Summary

Closes #784.

grafana_params.additional_dashboards is passed to plan.upload_files(...) in grafana_launcher.star. Per Kurtosis, upload_files accepts a GitHub locator, a path within the current package, or an absolute http(s) URL. Because that call executes inside ethpandaops/ethereum-package, a relative/local path supplied by a consumer package resolves against ethereum-package and fails:

'/src/grafana/tools/grafana/dashboards' doesn't exist in the package 'ethpandaops/ethereum-package'

This is inherent to Kurtosis's locator model — a relative consumer path cannot resolve against the consumer when the upload_files call lives in the dependency. The supported approach is an absolute github.com/<org>/<repo>/path locator (or an http(s) URL) pointing at your own repo.

Change

Documentation only — expands the additional_dashboards comment in README.md to spell out that entries must be a GitHub locator or absolute http(s) URL, and that local/relative paths won't work when inheriting the package.

grafana_params:
  additional_dashboards:
    - github.com/<your-org>/<your-repo>/path/to/dashboards