Improve executor_runner cmake instructions (#11773)

### Summary
There is a mandatory flag `DEXECUTORCH_BUILD_EXECUTOR_RUNNER` must be
turned on in order to build `executor_runner`, but this flag is not
explicitly set in our tutorial. Though the default of it is `ON`, it can
still be a UX issue when following the tutorial where the flag was
toggled `OFF` silently when building for other target like
`llama_runner`.

### Test plan
CI

Author: guangy10

backends/xnnpack/README.md

@@ -105,6 +105,7 @@ mkdir cmake-out
 cmake \
     -DCMAKE_INSTALL_PREFIX=cmake-out \
     -DCMAKE_BUILD_TYPE=Release \
+    -DEXECUTORCH_BUILD_EXECUTOR_RUNNER=ON \
     -DEXECUTORCH_BUILD_EXTENSION_DATA_LOADER=ON \
     -DEXECUTORCH_BUILD_EXTENSION_MODULE=ON \
     -DEXECUTORCH_BUILD_EXTENSION_TENSOR=ON \

docs/source/tutorial-xnnpack-delegate-lowering.md

@@ -154,6 +154,7 @@ mkdir cmake-out
 cmake \
     -DCMAKE_INSTALL_PREFIX=cmake-out \
     -DCMAKE_BUILD_TYPE=Release \
+    -DEXECUTORCH_BUILD_EXECUTOR_RUNNER=ON \
     -DEXECUTORCH_BUILD_EXTENSION_DATA_LOADER=ON \
     -DEXECUTORCH_BUILD_EXTENSION_MODULE=ON \
     -DEXECUTORCH_BUILD_EXTENSION_TENSOR=ON \

examples/xnnpack/README.md

@@ -38,6 +38,7 @@ mkdir cmake-out
 cmake \
     -DCMAKE_INSTALL_PREFIX=cmake-out \
     -DCMAKE_BUILD_TYPE=Release \
+    -DEXECUTORCH_BUILD_EXECUTOR_RUNNER=ON \
     -DEXECUTORCH_BUILD_EXTENSION_DATA_LOADER=ON \
     -DEXECUTORCH_BUILD_EXTENSION_MODULE=ON \
     -DEXECUTORCH_BUILD_EXTENSION_TENSOR=ON \