docs(README): Refresh with package list, local setup and link fixes

[m7kvqbe1]

Related issue

N/A

Overview

Refreshes the root README, which was last meaningfully updated in Feb 2025 and had broken links and notable gaps. Adds a packages table, real local-development steps, and tidies the badges/links.

Reason

The README didn't list the monorepo packages, never told contributors to run pnpm install or how to start Storybook, and contained broken/stale links.

Work carried out

• Added a Packages table covering all five @royalnavy/* packages
• Added proper Developing locally steps (clone → nvm install → corepack enable → pnpm install → run Storybook on :6006)
• Documented root scripts (build/lint/test/storybook:static) and the pnpm --filter pattern for package scripts
• Fixed the broken license badge link (was github.com/design-system/...)
• Switched Storybook links to https (the http URL 301-redirects)
• Removed dead classic project-board links (GitHub sunset classic Projects)
• Removed a broken Chromatic logo image; added a Conventions section (Conventional Commits, Husky, Prettier)

Developer notes

• All internal links verified to resolve (packages, docs/CONTRIBUTING.md, LICENSE, .nvmrc).
• The pnpm --filter @royalnavy/react-component-library storybook command is verified against the package's scripts.
• Package descriptions are accurate to each package's contents (e.g. design-tokens covers colour/spacing/typography/breakpoints).