High-level release highlights

Like pantsbuild, let's introduce a high-level release highlight section to each release note.These high-level release highlights may be continuously updated, and should be:

- [ ] copied into the front-matter of [the long_description of the wheel packages](https://packaging.python.org/en/latest/guides/making-a-pypi-friendly-readme/) (displayed in pypi.org)\* https://pypi.org/project/backend.ai/ – Currently it contains `README.md` only.
- This may require modification of [our setupgen pants plugin](https://github.com/lablup/backend.ai/blob/aac1e76657975e36847299e362bd7b7efb45b457/tools/pants-plugins/setupgen/register.py#L109-L117).

- [ ] linked in the notes of [GitHub releases](https://github.com/lablup/backend.ai/releases).
Examples:

- https://github.com/pantsbuild/pants/blob/main/docs/notes/2.22.x.md
- https://github.com/pantsbuild/pants/blob/main/docs/notes/2.21.x.md\* https://github.com/pantsbuild/pants/releases/tag/release_2.21.0

To make it easier to automate copy-and-pasting of them in the release pipeline, let's add a dedicated directory (suggested: `docs/release-notes/{24.03,24.09,...}.md`) in this mono-repo.

(cc: @kimjmin, @YEONFEEL96)

JIRA Issue: BA-116