Design system page for Strata

Design system is a new page for Strata, NinjaTrader’s design system.

• It lists the Strata component package and the Strata MCP for coding agents.
• It sets the rules for building UI: use Strata components, style with tokens by name, and do not restyle a component at the call site.
• It asks repositories that build UI with Strata to add the recommended config from @nt-ninjatrader/eslint-plugin-strata.
• It asks repositories that build UI with Strata to add Drift for visual regression checks.
• It routes requests and contributions through #ask-strata-nt and the Strata contributing guide.