docs: Document staging deployment aggregation strategy

[Copilot]

Adds comprehensive documentation for the dual deployment strategy (production + staging) to README.md in response to review feedback.

Changes

• Production Deployment: Documents deploy.yaml workflow triggered on master pushes → forrt.org
• Staging Deployment: Documents staging-aggregate.yaml workflow with PR aggregation strategy → staging.forrt.org
• Aggregation Strategy: Explains how all open non-draft PRs are automatically merged into a temporary branch, with conflicting PRs skipped and logged
• Workflow Mechanics: Documents queuing behavior (sequential builds, not concurrent), job timeouts (10-20min), and automated branch cleanup (retains 5 most recent)
• User Guidance: Clarifies that staging shows combined state of all compatible PRs, not individual PR previews
• Automated Reporting: Documents PR deployment comments (success/conflict status) and monthly deployment reports

The documentation enables contributors to understand why staging.forrt.org may show or exclude their PR changes based on merge compatibility with other open PRs.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.