docs: disclose SimilarWeb dynamic pricing

[eddiearc]

Summary

• add structured x-aisa-pricing metadata and visible pricing disclosure to all documented SimilarWeb operations
• add English and Chinese SimilarWeb cost-control guides with rates, row caps, formulas, and IMPORTANT warnings
• add an approval-first agent contract to Agent Quickstart and llms.txt: read docs, /v1/models, and MCP discovery first; present exact scope, formula, and documented maximum exposure; require explicit user approval before any SimilarWeb data call
• state that SimilarWeb data endpoints are never price discovery: no unapproved probe/test/just-check request and no invented quote API
• route llms.txt and pricing navigation to the guide, and gate future OpenAPI syncs with a pricing-contract check
• clarify that dynamic catalog values are estimate-required rather than fixed per-call quotes
• extend the isolated Mintlify overlay to validate both Quickstart policy pages alongside SimilarWeb pricing and endpoint docs

Validation

• python3 scripts/generate_similarweb_pricing_docs.py --check
• python3 scripts/consolidate_openapi.py --output /tmp/aisa-openapi-approval-policy.yaml
• python3 scripts/check_similarweb_pricing.py --generated /tmp/aisa-openapi-approval-policy.yaml
• python3 scripts/validate_similarweb_mintlify.py

No runtime billing, quote API, gateway behavior, or real SimilarWeb request is included.