doc: document MCP stdio servers and CORS defaults in the server README [no release] [no ci] (#26847)
* doc: document MCP stdio servers and CORS defaults in the server README
The MCP arguments were listed but nothing explained what an MCP server
is or how to declare one. Cover the stdio transport, the config keys,
the tool naming, and add a POSIX shell echo server as a minimal
example.
Also document the CORS behavior: the default reflected origin, the
switch to localhost once tools are enabled, and the recommended setting
per deployment.
* doc: drop the inline MCP shell example from the server README
The example parsed JSON-RPC by hand and sat in a page people copy paste
from, into servers spawned with the privileges of llama-server. Point to
the specification instead.
Link the pull request that introduced the feature, and keep a short
mcp.json snippet so the table of configuration keys has a declaration to
refer to.