docs: improve documentation navigation and reading order

[Copilot]

The repository already had the core design and product docs, but they were harder to discover and harder to approach in the right order. This change makes the documentation entry points more legible so contributors can get to the right source quickly.

• What changed

  • Added docs/README.md as a documentation index for:
    • recommended reading order
    • task-oriented document lookup
    • quick mental model of where product, architecture, development, and UI guidance live
  • Updated the root README.md to:
    • surface the new docs guide
    • expose the main documentation entry points
    • clarify the intended reading order for UI-related docs

• Why this helps

  • Reduces time spent guessing which document to read first
  • Makes the existing UI contract easier to understand as a connected set of docs instead of isolated files
  • Improves onboarding without introducing another parallel documentation structure

• Documentation map

  ## Documentation guide
  
  - Documentation guide → docs/README.md
  - Development workflow → docs/DEVELOPMENT.md
  - Backend architecture → docs/ARCHITECTURE.md
  - UI foundation → docs/design/ui-foundation.md
  - Component contracts → docs/design/component-contracts.md
  - UX language → docs/product/ux-language.md