Update project documentation to match template.

[SHession]

What does this change?

Using the recommendations template and Claude for a first pass, I have generated some improved documentation for this project.

It should cover all the basic points and make the tool easier to reason about for newcomers. There is some guidance on writing effect documentation here: https://github.com/guardian/recommendations/blob/main/documentation.md#readme-template

Reviewers should be transparent about areas they found confusing or irrelevant. Identifying these areas in the documentation is the most helpful feedback an author can receive. It allows an author to consider their real audience and work with them to provide the right level of detail.

How can we measure success?

The tool is easier to understand for newcomers.

[Copilot]

Pull request overview

Updates the project’s README to follow the Guardian recommendations template, making the library’s purpose, onboarding steps, and supporting docs easier to navigate for new contributors and consumers.

Changes:

• Replaces the existing README with a structured, templated format (intro/getting started/how it works/links/terminology).
• Adds/organizes links to existing docs (quick-start, how-it-works, vision, ADRs) and adds a terminology glossary.
• Expands local dev, testing, releasing, and local-consumption guidance (changesets + yalc).

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