Add towncrier for managing changelog

[ItsDrike]

This PR adds towncrier as a tool for managing changelog of the project.

This means a few changes to the project for contributors:

• All PRs will now have to include a changelog fragment, stating what they've changed, enforced by a new fragment-check workflow
  • Although if it's a small enough chage, the PRs author can request for this fragment check to be skipped (using a label added by a maintainer)
• Once a new release is expected to be made, the maintainers are expected to manually trigger the new prepare-release workflow, which will automatically create the changelog from individual fragments, and remove the fragments. Only after that should a new release be made (using the standard method - pushing a release tag to main branch)

For more info, a readme file was added: changes/README.md, explaining how this process works, along with a link to the towncrier documentation. This file was then also mentioned as required reading in CONTRIBUTING.md.