data: add Evidently AI startup offer

[itzroberl]

What changed

Adds Evidently AI and its Evidently Cloud for AI startups program as one consolidated startup offer. The record captures the published Expert tier credits, up to 90% discount for six months, included evaluation/testing features, priority support, and eligibility rules.

Closes #672

Sources

• https://www.evidentlyai.com/sign-up-startups

Certification

• I changed only allowed repository content and did not mix vendor YAML with other paths.
• Public sources substantiate every submitted factual value; any Sourcey-written prose is a neutral summary, not a fabricated vendor quote.
• I did not author verification, provenance, freshness, or signature claims.
• My commits include a Signed-off-by line.