docs: tune Mintlify search metadata

[sorinc03]

Fixes #11.

Summary

• adds supported Mintlify search ranking boosts in docs.json so onboarding and guides rank above reference pages
• adds page-level keywords across every MDX page, including Stellar/Soroban/friendbot/testnet synonym coverage
• enables telemetry explicitly and adds a search review cadence plus SEARCH_TESTS.md expected-query checklist
• fixes broken relative SDK links found while validating

Notes

Mintlify current public schema supports search.prompt, navigation/frontmatter boost, and page keywords, but not a schema-level docs.json synonym map. I implemented synonym coverage through indexed page keywords, which is the supported internal-search path.

Validation

• npx --yes prettier@3.5.3 --check docs.json '*.mdx' 'architecture/*.mdx' 'api-reference/*.mdx' 'contracts/*.mdx' 'guides/*.mdx' 'sdk/*.mdx' 'sdk/chains/*.mdx' CONTRIBUTING.md SEARCH_TESTS.md
• npx --yes mintlify@latest validate
• npx --yes mintlify@latest broken-links