Docs: Add comprehensive Cmux documentation (features, settings, local vs cloud)

[notkainoa]

Is your feature request related to a problem? Please describe.
It’s hard to discover and understand Cmux’s full functionality without a single, organized docs page. New and existing users need a clear reference for features, settings, and workflows, plus guidance. For example I have no idea what cloud mode is and what the difference is from local mode, or how it works.

Describe the solution you'd like
A dedicated, structured documentation site or page covering:

• Overview: What Cmux is, key use cases, supported platforms.
• Quick Start:
  • Local Mode: install, init, start, environment requirements.
  • Cloud Mode: account setup, auth, regions, quotas.
• Core Features:
  • Multiplexing behavior and routing
  • Session management
  • Models, tools, and connectors supported
  • Project/workspace structure
  • CLI commands and flags
• Configuration & Settings:
  • Global config (paths, env vars)
  • Per-project config
  • Logging, telemetry, and privacy controls
  • Resource limits and performance tuning
• Local vs Cloud mode:
  • Feature parity matrix
  • Data flow diagrams
  • Persistence, caching, and storage differences
  • Security considerations (keys, secrets, isolation)
  • Pricing/limits (if applicable) and recommended use cases
• Integrations:
  • Git, package managers, and build systems
  • Model providers and API keys
  • Webhooks and event emitters
• Troubleshooting:
  • Common errors and fixes (e.g., “git is missing”, EventEmitter memory leak, lingering processes after ctrl‑C)
  • Diagnostic commands and logs
• Guides & Recipes:
  • Example setups for local dev
  • Example cloud deployments
  • Custom models and extensions
• Reference:
  • CLI command reference with examples
  • Config schema
  • API endpoints (if public)
• Versioning & Changelog:
  • How docs map to releases
  • Known issues and deprecations

Describe alternatives you've considered

• Scattered README sections and issue threads: helpful but incomplete and hard to navigate.
• Inline comments in code: good for contributors, not ideal for users.

Additional context

• Users are currently opening issues about missing git, memory leaks, and process persistence. A Troubleshooting section with known problems and steps would reduce repeat reports.
• A feature parity table for local vs cloud mode would help users choose the right setup quickly.
• Suggest hosting: docs/ in repo plus a generated site (using something like Docusaurus) to enable search, versioning, and deep links.