Root README updated for HelpGenerator

[jeffpatton1971]

Acceptance criteria:

• Root README.md updated to:

  • Project name: HelpGenerator
  • Scope + non-goals (no PlatyPS, no hosting, no prose rewriting)
  • Outputs (MAML, HelpInfo.xml, Markdown, CAB layout)
  • Links to docs/usage/quickstart.md and docs/adr/README.md