docs: make table of contents collapsible and improve quick start

- Wrap TOC in collapsible details/summary tags to reduce initial scroll
- Update quick start example to use streamable HTTP instead of stdio
- Reorder overview to emphasize servers over clients
- Move resources section after tools since tools are more commonly used
- Add structured output examples to tool responses
- Improve code examples with better formatting and comments
- Add direct links to MCP clients (Inspector, Claude Code, VS Code, Cursor)
- Clarify tool vs resource use cases
- Remove redundant 'What is MCP?' section
- Simplify stateless HTTP example with clearer comments

Author: domdomegg