docs(mcp-auth): revamp README get started and add opaque token example

[xiaoyijun]

Summary

Revamp the README to spotlight the mcp-auth surface and document how to verify opaque access tokens.

• Add a docs & samples quick-links row under the tagline
• Replace the two usage examples with a single trimmed "Get started" walkthrough (MCPAuth declaration, getBearerAuthOptions with requiredScopes, getAuthInfo with the per-tool scopes hint), pointing to mcp-auth.dev tutorials and the sample servers for complete projects
• Add an "Opaque access tokens" section: a complete OAuthTokenVerifier implementation against an RFC 7662 introspection endpoint, reusing the metadata half of mcp-auth (resourceMetadataUrl, discovery documents, and the McpAuthInfo shape so getAuthInfo() keeps working)
• Drop provider-list links pending the website revamp

Testing

Tested locally.

Checklist

• .changeset
• unit tests
• integration tests
• necessary TSDoc comments

🤖 Generated with Claude Code

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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