Introduce Changesets for auto-generated changelogs

[mjakl]

The repository uses conventional commits, but the granularity of a commit does not sufficiently match a significant change worth documenting.

The Backstage project itself also uses conventional commits (not very consistently, though), but relies on Changesets to generate changelogs (after the first stable release).

It would be good to introduce Changesets, and include a changeset for each significant change in the code base. During pull request reviews, thes changeset can be ensured/reviewed/discussed.

Conventional commits could stay if they serve a purpose (or just for consistency).