docs: update stale contributor setup and architecture in docs/README.md

[vishxrad]

Problem

docs/README.md no longer reflects the current repository layout or documentation site.

The README currently:

• says the docs live at js/packages/docs-v3 with a repo/docs-v3 symlink
• instructs contributors to run cd js && pnpm install
• references the removed @openuidev/react-core package
• documents old route groups and content sections such as (design-system), introduction, and generative-ui
• describes a docs-v3/ project tree that does not match the current docs/ directory
• lists navigation tabs that no longer match the current navbar

This makes the contributor setup instructions fail and gives maintainers an inaccurate mental model of the docs application.

Proposed update

Rewrite docs/README.md against the current repository structure and workspace configuration.

Acceptance criteria

• Setup commands work from the repository root using the current pnpm workspace.
• The README identifies the site location as docs/.
• Package references match docs/package.json, including @openuidev/lang-core, @openuidev/react-lang, @openuidev/react-headless, @openuidev/react-ui, and @openuidev/cli.
• The project tree reflects the current app/, content/docs/, components/, shared/, and generated-content structure.
• Documentation sections and navbar descriptions match the current OpenUI, Chat, and API Reference organization.
• Instructions for adding MDX pages and component/design-system pages use current paths.
• All references to docs-v3, js/packages, the symlink, and @openuidev/react-core are removed.

[shogun444]

Hi @vishxrad , I'd like to take this one.

I'll update docs/README.md to match the current repository structure, workspace setup, and documentation architecture.