Write a blogpost to announce LLM evaluation support in Haystack

[julian-risch]

• [ ] Colab code works as expected
• [ ] Colab has view access for the general public
• [ ] Colab has been added to cookbook repository
• [ ] Blog post has been reviewed by someone else on the dev rel team, all feedback addressed
• [ ] all relevant code snippets from Colab also exist in blog post
• [ ] documentation pages and other relevant links has been added
• [ ] blog post has catchy header image (can use Midjourney for this)
• [ ] all blog post images have alt text / captions
• [ ] blog post metadata is correct (author, title, subtitle, etc)
• [ ] blog post preview displays, do a pass through that to check for typos etc
• [ ] social && promotion:
  • [ ] LinkedIn
  • [ ] Twitter
  • [ ] deepset Slack
  • [ ] Haystack discord
  • [ ] Reddit (optional)
  • [ ] Medium crosspost (optional), set canonical link to Haystack blog
  • [ ] dev.to crosspost (optional) set canonical link to haystack blog
  • [ ] external Slacks (optional)