feat: Amplify Gen 2 CDK backend — CloudFront + Lambda for Accept: text/markdown

[Nonnyjoe]

Summary

Adds the Amplify Gen 2 CDK backend that powers HTTP content negotiation for the docs site.

• amplify/functions/markdown-negotiation/ — Amplify-managed Lambda (docs-markdown-negotiation) visible in Amplify Console → Functions. Rewrites Accept: text/markdown requests to .md paths and proxies to the Amplify origin.
• amplify/custom/markdown-negotiation/ — CDK construct that creates a CloudFront distribution with the Lambda Function URL as its origin. Cache policy varies on the Accept header so HTML and Markdown variants cache separately.
• amplify/backend.ts — wires Lambda and CloudFront together, attaches a Function URL, passes the hostname to the CloudFront construct, and emits the distribution domain as a CloudFormation output.
• amplify.yml — adds the backend.phases.build CDK deploy step alongside the existing frontend build. Also includes rewrite rules and CDN cache headers (superset of what feat/llm-readability added).
• Removes package-lock.json (project uses yarn); adds Amplify Gen 2 and AWS CDK dependencies to package.json and yarn.lock.

Dependencies

This PR is currently deployed and being monitored on staging. Merge feat/llm-readability (#329) first.

Infrastructure notes

• CloudFront distribution domain is emitted as MarkdownNegotiationDomain in the MarkdownNegotiationStack CloudFormation stack.
• DOCS_DOMAIN env var drives both the CloudFront alternate domain and llms.txt base URLs. Defaults to staging.docs.cartesi.io when unset.
• DOCS_CERT_ARN is optional — omit for staging (no custom domain), set to an ACM cert ARN (us-east-1) for production.

Test plan

• Amplify build completes with backend CDK deploy phase
• docs-markdown-negotiation Lambda visible in Amplify Console → Functions
• MarkdownNegotiationStack appears in CloudFormation with CREATE_COMPLETE or UPDATE_COMPLETE
• curl -H "Accept: text/markdown" https://docs.cartesi.io/cartesi-rollups/overview returns 200 OK with Content-Type: text/markdown
• Normal browser request to https://docs.cartesi.io/cartesi-rollups/overview returns HTML (no regression)

Closes #328