Add section: "Core concepts"

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Adds a new "Core concepts" section with four explanation pages:

• What is Lightpanda — what Lightpanda is, why it's built from scratch instead of forking Chrome/Blink/WebKit, and how it fits into a browser automation stack.
• Architecture overview — how the layers fit together: from the control surface (CDP, agent, MCP) to the browser engine (V8, Web APIs, HTML parser, network layer)
• Benchmarks — Lightpanda's three published benchmarks against headless Chrome (crawling, page automation, AI agent workloads).
• When to use local vs cloud — how to decide between running Lightpanda locally vs on Lightpanda Cloud, comparing CDP, MCP, agent, HTTP API, proxy, and telemetry coverage across both.

Registers the new section and pages in _meta.ts.