Automatically add release notes to Haystack website

[julian-risch]

Haystack release notes are currently added to the Haystack website manually https://haystack.deepset.ai/release-notes

We should automate this step so that for example https://haystack.deepset.ai/release-notes/2.3.1 is automatically added to the website when the v2.3.1 release is triggered. The generated release is then also available from https://github.com/deepset-ai/haystack/releases/tag/v2.3.1