Add default /docs route to list information about the HTTP MCP server

Is your feature request related to a problem? Please describe.
Would be useful to have an equivalent to something like how FastAPI generates OpenAPI docs and are automatically accessible at /docs route.

Describe the solution you'd like
Add a /docs route to the starlette app that provides information about the mcp server (available tools, prompts, resources, version, etc.)

Describe alternatives you've considered
none

Additional context
Basic example I currently use:

该 issue 没有指定文件或测试；首先定位 Starlette 应用及其现有的 HTTP 路由设置。围绕 MCP 服务器的工具、提示、资源和版本定义 /docs 响应，然后验证该路由是否公开预期的信息。