Document all free-tier capabilities in a single markdown file

[petterlindstrom79]

The 5 free-tier capabilities (iban-validate, email-validate, dns-lookup, json-repair, url-to-markdown) are mentioned in various READMEs but there's no single reference document showing their input schemas, example outputs, and rate limits.

What to do:

• Create docs/free-tier.md with: slug, description, input schema, example input, example output, rate limits
• Include curl examples that work without any API key
• Link from the main README

Good first issue — documentation only, no code changes.