[CI] Add release-notes automation

`docs/release-notes` ought to be updated with each release, but this process has been done manually in the past.
Let's add a github action that creates these notes based off the latest release notes.

This tool can be used : https://elastic.github.io/docs-builder/contribute/changelog/
point of contact : @lcawl

Start by inspecting docs/release-notes and the repository's existing GitHub Actions workflows. Read the linked docs-builder changelog guidance and identify how the latest release notes should be supplied to the tool. Done means a release triggers an action that creates or updates the corresponding release notes without the current manual process.

Written by the indexing model from the issue text.