chore(components): add development documentation

[thomasguillot]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Related: #4486 and #4487

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If some of the documentation is incorrect, feel free to update

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Adds a comprehensive DEVELOPMENT.md for the components package to guide internal developers and AI tools. This is probably not perfect but I think this is a starting point.

What's in the guide

• Purpose & when to use – When to use Newspack components vs @wordpress/components (backend wizards, blocks, frontend) and an "in short" summary.
• Component selection hierarchy – Step-by-step: Newspack first → WordPress → designer review for new components.
• Design & layout at a glance – Spacing scale (8px unit), when to use HStack/VStack/Grid, hierarchy (SectionHeader → Card → ActionCard), breakpoints, and pointer to full Styling section.
• Available components – Categorized list (layout, form, content, wizard, etc.) with design/usage notes where relevant.
• Usage – By context (backend, blocks, frontend) and common patterns (Wizard, withWizardScreen, withWizard, hooks, AutocompleteTokenField in blocks) with code examples.
• Import patterns – Monorepo (relative paths) vs npm (newspack-components), alphabetical named imports, no namespace import, Router proxy.
• Common WordPress components – Reference list (form, layout, panels, etc.), icons (WordPress/icons then newspack-icons, no Dashicons), experimental APIs with eslint-disable-line.
• Styling – Naming (BEM-ish), spacing scale table, layout guidance, responsive breakpoints, visual hierarchy patterns, component states.
• Contributing – Component patterns (wrapping vs standalone) and development guidelines.

Benefits

• Single source of truth for how and when to use components in wizards/settings.
• Design system in one place – Spacing, hierarchy, breakpoints, and states documented.
• AI-friendly – Clear, consistent instructions for both humans and AI (selection, imports, patterns).

How to test the changes in this Pull Request:

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Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?