Instructions for changelog entries

### Problem

[`CONTRIBUTING.md`](https://github.com/dbt-labs/dbt-agent-skills/blob/main/CONTRIBUTING.md) doesn't give any instructions relating to changelog entries.

### Solution

Choose one of the following:
1. Add instructions to `CONTRIBUTING.md` for `changie new`, or
2. Remove Changie related stuff like [`CHANGELOG.md`](https://github.com/dbt-labs/dbt-agent-skills/blob/main/CHANGELOG.md), [`.changie.yaml`](https://github.com/dbt-labs/dbt-agent-skills/blob/main/.changie.yaml) and [`.changes`](https://github.com/dbt-labs/dbt-agent-skills/tree/main/.changes)