fix release notes link in project metadata

[pmeier]

1. We renamed the file.
2. We should point at the release notes of the current stable release rather than the dev version.

[smokestacklightnin]

The link works for me but it the page doesn't render properly for me. It gives a 404 not found response and the rendering looks broken

[pmeier]

Sorry for not providing enough context. This is similar to #343. The link will work after the 0.2.0 release. Right now the correct link is https://ragna.chat/en/latest/references/release-notes/. When the release is published, what currently is in /latest will be moved to /stable and thus making the link in this PR valid.

However, in contrast to #343, this link is only in the pyproject.toml rather than in the very visible README. Thus, we can merge this already and don't need to wait until the last second.