feat: restructure Getting Started section with SEO optimization

- Create new getting-started/ folder with 4 pages:
  - about.mdx: Project origin, original 33 concepts, what we changed
  - how-to-learn.mdx: Page structure, resource types, learning tips
  - prerequisites.mdx: Tools setup (browser, code editor, Node.js)
  - learning-paths.mdx: Beginner, intermediate, interview prep paths

- Refactor index.mdx as Welcome page:
  - Add SEO-optimized title and description
  - Add intro paragraph with keyword placement
  - Add Web Platform category card (7 categories total)
  - Update links to new getting-started pages

- Delete old introduction.mdx (content moved to new pages)

- Update docs.json navigation for new structure

- SEO improvements across all pages:
  - Titles under 60 characters with hooks
  - Descriptions 140-160 characters with keywords
  - Question-format H2s for featured snippets