[Doc] Update Ruby client example to use explicit connect

## Motivation and Context

Follow-up to modelcontextprotocol/ruby-sdk#336 and
modelcontextprotocol/quickstart-resources#133.

The Ruby example in `docs/docs/develop/build-client.mdx` constructed `MCP::Client`
with a stdio transport and immediately called `client.tools`, relying on the implicit
`initialize_session` that the stdio transport performed on the first request.

ruby-sdk PR modelcontextprotocol#336 added `MCP::Client#connect` for the stdio transport so that
the explicit `initialize` plus `notifications/initialized` handshake is exposed
at the public API level, matching the Python SDK (`ClientSession.initialize()`)
and the TypeScript SDK (`Client.connect(transport)`). The implicit-init path is
preserved as a backwards-compatible shim, but new code is expected to call `connect` explicitly.

This change updates the Ruby example so readers learn the explicit `@mcp_client.connect`
pattern from the start, consistent with how the Python and TypeScript examples present
the handshake.

## How Has This Been Tested?

- Re-read the surrounding `connect_to_server` example in
  `docs/docs/develop/build-client.mdx` to confirm `@mcp_client.connect`
  fits naturally between the client construction and the subsequent
  `@mcp_client.tools` call.
- Ran `npm run prep` to confirm there are no documentation warnings or
  errors introduced by the change.

## Breaking Changes

None. This is a documentation-only change that adds a single explicit `connect` call to
the Ruby client example. No schema, navigation, or runtime behavior is affected.

Author: koic

docs/docs/develop/build-client.mdx

@@ -1791,6 +1791,7 @@ def connect_to_server(server_script_path)
 
   @transport = MCP::Client::Stdio.new(command: command, args: [server_script_path])
   @mcp_client = MCP::Client.new(transport: @transport)
+  @mcp_client.connect
 
   tool_names = @mcp_client.tools.map(&:name)
   puts "\nConnected to server with tools: #{tool_names}"