docs(readme): improve product landing experience - #6

Summary

• Turn the README header into a clearer product landing section
• Clarify the dataset value proposition, trust signals, and primary use cases
• Add a tracked link to the website dataset landing page
• Add UTM attribution to the remaining untracked AICostBudget website link
• Preserve the UTM links fixed in PR fix(readme): correct GitHub UTM attribution #5
• Preserve provider links and public API download URLs

README changes

• Stronger one-sentence value proposition
• Current coverage and verification signals from meta.json
• Clear primary and secondary calls to action
• Short trust and capability summary
• Complete GitHub referral attribution for AICostBudget website links

Validation

• README.md is the only changed file
• Existing six tool UTM links remain intact
• Existing provider UTM links remain intact
• No Hugging Face attribution remains in the GitHub README
• All AICostBudget website links have source, medium, campaign, and content
• Public JSON, CSV, and model API URLs remain unchanged