[9.1] (backport #9865) Add ingest-docs team as CODEOWNERS for release notes and docset.yml

[mergify]

Related to #9440

Adds ingest-docs team as CODEOWNERS for docs/release-notes and docset.yml.

In my experience, someone with admin rights in this repo will need to add the @elastic/ingest-docs team with write permissions even if all of elastic already has write access.

When the code owner is a team, that team must be visible and it must have write permissions, even if all the individual members of the team already have write permissions directly, through organization membership, or through another team membership.

From: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners

---

This is an automatic backport of pull request #9865 done by Mergify.

[colleenmcginnis]

Backporting to 9.1 so the ingest-docs will be tagged to review PRs like #9927.