add blog for retrieval evaluation metrics

[iamleonie]

What's being changed:

Add blog post for retrieval evaluation metrics

Type of change:

• Documentation updates (non-breaking change to fix/update documentation)
• Website updates (non-breaking change to update main page, company pages, pricing, etc)
• Content updates – blog, podcast (non-breaking change to add/update content)
• Bug fix (non-breaking change to fixes an issue with the site)
• Feature or enhancements (non-breaking change to add functionality)

How Has This Been Tested?

• Github action – automated build completed without errors
• Local build - the site works as expected when running yarn start

note, you can run yarn verify-links to test site links locally