✨📝 Add changelog and upgrade guide infrastructure

[burgholzer]

Description

This PR adds a dedicated changelog and upgrade guide to the project as a centralized place to keep track of significant updates to this library.
Both of these files are curated selections of the PRs that go into a release.
Notably, these are written for humans and not for machines.
As such, they are not automatically scraped together like the drafts created by the release drafter action. These drafts may still serve as blueprints for corresponding changelog entries.

Checklist:

• The pull request only contains commits that are related to it.
• I have added appropriate tests and documentation.
• I have made sure that all CI jobs on GitHub pass.
• The pull request introduces no new warnings and follows the project's style guidelines.