Contributors shouldn't edit the CHANGELOG.md directly anymore

[dbeatty10]

Describe the documentation change

We should clarify within our contributing guide that users should not make modifications to CHANGELOG.md anymore, because they will be overwritten during the release process.

While dbt-codegen does not use changie like dbt-core, et al. it also doesn't need manual updating by users anymore.

The implementation can use dbt-core or dbt-snowflake for inspiration.

Additional context

This is the same thing as dbt-labs/dbt-utils#914, but applied to the dbt-codegen repo.

Who will this benefit?

This will benefit contributors and maintainers -- less to do all-around.