Rewrite and expand README documentation

[netmagik]

Description

• New "When to Use This Plugin" section — comparison table explaining when to use field-level vs page-level experiments, helping users choose the right approach

• New "Page-Level Experiments" section — step-by-step guide covering plugin configuration with a reference field, creating a route experiment document type, querying the correct page, and implementing a proxy for routing

• New "Variant Assignment" section — documents cookie-based assignment, variant ID consistency, reading variants in page components, and third-party integration patterns

• Improved examples throughout — replaced generic placeholder examples (e.g., experiment 1, 123-a) with realistic, self-documenting ones (e.g., Homepage Headline Test, control, Emotional Appeal)

• Expanded "Overwriting field names" — added a concrete audience segmentation example with stored data structure and query patterns

• Improved "Split testing" section — renamed to "Split testing (URL-based)" with complete working code examples including cookie handling

• Added visual aids — beaker icon SVG for inline rendering, screenshots showing the field-level and page-level experiment UIs (field-experiment.png, page-experiment.png)

• Added links to the example repository and a Loom video walkthrough

• Fixed typos and formatting — "know" to "known", "defined" to "define", "intto" to "into", fixed broken markdown anchor links, removed duplicate License entry from TOC

• Added new files: beaker.svg — icon used inline in the README to explain the experiment picker,
  field-experiment.png — screenshot of the field-level experiment UI,
  page-experiment.png — screenshot of the page-level experiment UI

What to review

Testing