feat: Add comprehensive references and sidebar component for AI SDK UI

- Create `mastra-reference.md` to document the Mastra backend structure, including agents, tools, networks, workflows, configuration, A2A coordination, MCP server, evaluations, services, data processing, and policy definitions.
- Create `reference.md` to provide an extended reference for AI elements, agent configuration schema, and API routes for the dashboard, linking to the newly created `mastra-reference.md`.
- Implement `MainSidebar` component in `main-sidebar.tsx` to enhance the user interface with navigation for agents, tools, workflows, and observability features.
  - Fetch and display agents, tools, workflows, traces, threads, and vector indices from the backend.
  - Include collapsible sections for better organization of sidebar items.
  - Add functionality to select agents and create new threads directly from the sidebar.