chore: add per-package changelog files

Status Quo: We currently have a single `docs/CHANGELOG.md`, which is a pre-weld leftover from execution-spec-tests.

Proposal: Add separate changelog files for each area/package in the repository:

- `src/ethereum/CHANGELOG.md` - ethereum package changes.
- `packages/testing/CHANGELOG.md` - testing workspace changes (not essential now, forward thinking).
- `tests/CHANGELOG.md`:
- extremely helpful for test releases and client devs.
- TBD: Should this only list changes that change test fixtures? i.e., skip refactors, `feat`=new test, `fix`=change in test behavior.

## Motivation

The test changelog is essential to inform client devs which tests have changed, which new tests are included, and what to expect from release to release.

## Drawback

The `tests/` changelog will likely have frequent merge conflicts, especially when dealing with branches that create test releases (e.g., the `devnets/amsterdam/2` branch). This is largely unavoidable given the nature of concurrent test development.

## Alternative Approaches

We could automatically generate a changelog based on commits.