Add local-development docs scaffold under root Documentation

[Copilot]

Adds first-class onboarding docs for running Lens locally by introducing a DocFX-style Documentation/ tree and a focused getting-started path for local development.

• Documentation root + navigation

  • Added Documentation/index.md and Documentation/toc.yml as the site entrypoint.
  • Wired section navigation to GettingStarted/toc.yml.

• Getting Started section

  • Added Documentation/GettingStarted/index.md and section-level toc.yml.
  • Introduced a dedicated LocalDevelopment topic wired through nested TOCs.

• Local development guide

  • Added Documentation/GettingStarted/LocalDevelopment/index.md covering:
    • prerequisites
    • install/build flow
    • loading unpacked extension from Source/dist
    • Arc base URL and tenant header configuration
    • watch-mode workflow for iterative changes

# Documentation/toc.yml
- name: Lens Documentation
  href: index.md
  items:
    - name: Getting Started
      href: GettingStarted/toc.yml