Improve README onboarding and contributor guidance

Summary

The root README.md already has good high-level content, but it could do a better job of onboarding both users and contributors. Important information is present, but some of it is scattered, duplicated, or missing from the first-read experience.

Problems observed

• The README does not state prerequisites up front (for example, the repository currently targets Go 1.24.11 in go.mod).
• The project’s scope and limitations are implied, but not summarized early for readers who may expect a full interactive browser.
• The "Quick Start", testing, and WebAssembly instructions are spread across multiple sections/documents without a concise task-oriented flow.
• The testing command appears under the screenshots section, which makes the document harder to scan.
• Contributor-specific expectations (for example where to find milestones/testing docs and when screenshots should be refreshed for rendering changes) are not surfaced clearly in the main README.

Suggested improvements

• Add a short prerequisites section near the top.
• Add a brief "What this project is / is not" summary.
• Reorganize quick-start content into clearer tasks:
  • build the CLI
  • render a local file
  • render a remote URL
  • run tests
  • run the WASM demo
• Move testing instructions into their own section.
• Add a short contributor section linking to MILESTONES.md, TESTING.md, and screenshot expectations.
• Review the README for duplicated or overly long sections and tighten the flow.

Acceptance criteria

• A new reader can understand the project scope within the first screenful of the README.
• A contributor can find the main build/test/demo commands quickly.
• The README clearly points to deeper docs without forcing readers to hunt through the repo.
• Testing and contributor guidance are in logically named sections.