[DOCS] Add changelog config file

[lcawl]

This PR used the changelog init command to create a configuration file that can be used to generate release note docs.

The product values that are used in the changelogs must exist in https://github.com/elastic/docs-builder/blob/main/config/products.yml so this PR must be merged after elastic/docs-builder#2979

NOTE: I couldn't find a label to use for the ">release_notes:skip" scenario where you know you don't want a release note, so if you add one we should update this config file in the rules.create section.

To test it out:

1. Install docs-builder per https://www.elastic.co/docs/contribute-docs/locally
2. Run the following command in this agent-skills repo:

   docs-builder changelog add --prs 21

   The command will fail because it doesn't have a "type" label. You can fix the latter by adding a "type" label to the PR and/or re-running the command with an explicit --type:

   docs-builder changelog add --prs 74 --type bug-fix