data: add Rhetorik startup credits

[villanelle8-byte]

Closes #775

What changed

• Added a new Rhetorik vendor record.
• Recorded the current Data for Startups offer: $10,000 in free API data credits for proof-of-concept or MVP development, followed by discounted access at launch and growth stages.
• Captured the published startup eligibility and public application route.

Source

• https://rhetorik.com/lp-startups/

Validation

• Ran the repository's exact digest-pinned Sourcey Catalog Verifier against this change.
• Result: {"status":"valid","vendors":1,"programs":1,"offers":1}

AI disclosure

AI assisted with source discovery and drafting. The submitted facts were reviewed against the cited first-party page before submission.

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 commit includes a Signed-off-by line.