Fix instructions for running conformance tests

- The server-servlet app depends on mcp, which must be either installed
  or included in the compile artifact for `mvn exec:java` to pick it up.
- Change the build instructions to build from root, use a `-pl` flag to
  target the servlet app.
- Disable the exec plugin from the root pom.

Signed-off-by: Daniel Garnier-Moiroux <git@garnier.wf>

conformance-tests/server-servlet/pom.xml

@@ -65,6 +65,7 @@
 				<version>3.1.0</version>
 				<configuration>
 					<mainClass>io.modelcontextprotocol.conformance.server.ConformanceServlet</mainClass>
+					<skip>false</skip>
 				</configuration>
 			</plugin>
 		</plugins>