Update instruction to install flatc (#447)

Summary:


See context in D49487908, the `flatc` from `brew install flatbuffers` seems good.

Differential Revision: D49530021

Author: cccclai

docs/website/docs/tutorials/00_setting_up_executorch.md

@@ -15,9 +15,7 @@ To install conda, you can look at the
 ```bash
 conda create -yn executorch python=3.10.0
 conda activate executorch
-
 conda install cmake
-conda install -c conda-forge flatbuffers
 ```
 
 ### Step 2: Clone the `executorch` repo
@@ -51,7 +49,22 @@ you may need to re-install a new version the PyTorch nightly pip package. The
 `TORCH_VERSION` value in this document will be the correct version for the
 corresponsing version of the repo.
 
-### Step 4: Generate a program file from an `nn.Module`
+### Step 4: Install host built tool
+
+Install [flatc V23.5.9](https://github.com/google/flatbuffers/releases/tag/v23.5.9) as it's prerequist for both AOT and runtime.
+There are several ways to get the flatc compiler, including
+- [MacOS only] `brew install flatbuffers`
+- Download a prebuilt binary from https://github.com/google/flatbuffers/releases/tag/v23.5.9
+- Build it from `executorch/third-party/flatbuffers`, [following instruction from the official website](https://github.com/google/flatbuffers#quick-start), for example, if it's on MacOS:
+```bash
+cd executorch/third-party/flatbuffers && cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release && cmake --build . -j 9
+```
+Add the flatc path to $PATH like
+```
+export PATH="/path/to/flatc:$PATH"
+```
+
+### Step 5: Generate a program file from an `nn.Module`
 
 Via python script:
 ```bash

docs/website/docs/tutorials/cmake_build_system.md

@@ -123,10 +123,10 @@ rm -rf cmake-android-out && mkdir cmake-android-out && cd cmake-android-out
 
 # point -DCMAKE_TOOLCHAIN_FILE to the location where ndk is installed
 # Run `which buck2`, if it returns empty (meaning the system doesn't know where buck2 is installed), pass in pass in this flag `-DBUCK2=/path/to/buck2` pointing to buck2
-cmake -DCMAKE_TOOLCHAIN_FILE=/Users/{user_name}/Library/Android/sdk/ndk/25.2.9519653/build/cmake/android.toolchain.cmake  -DANDROID_ABI=arm64-v8a ..
+cmake -DCMAKE_TOOLCHAIN_FILE=/Users/{user_name}/Library/Android/sdk/ndk/25.2.9519653/build/cmake/android.toolchain.cmake  -DANDROID_ABI=arm64-v8a -DBUCK2=/path/to/buck2 -DFLATC_EXECUTABLE=/path/to/flatc ..
 
-cd  ..
-cmake --build  cmake-android-out  -j9
+# Move to the executorch folder and build
+cd  .. && cmake --build  cmake-android-out  -j9
 
 # push the binary to an Android device
 adb push  cmake-android-out/executor_runner  /data/local/tmp/executorch
@@ -162,7 +162,10 @@ git clone  https://github.com/leetal/ios-cmake.git
 rm -rf cmake-ios-out && mkdir cmake-ios-out && cd cmake-ios-out
 
 # change the platform accordingly, please refer to the table listed in Readme
-cmake . -G Xcode -DCMAKE_TOOLCHAIN_FILE=~/ios-cmake/ios.toolchain.cmake -DPLATFORM=SIMULATOR
+cmake -G Xcode -DCMAKE_TOOLCHAIN_FILE=~/ios-cmake/ios.toolchain.cmake -DPLATFORM=SIMULATOR  -DBUCK2=/path/to/buck2 -DFLATC_EXECUTABLE=/path/to/flatc -DPYTHON_EXECUTABLE=/path/to/miniconda/envs/executorch/bin/python3 ..
+
+# Move to the executorch folder and build
+cd .. && cmake --build  cmake-ios-out  -j9
 
 # Create an include folder in cmake-ios-out to include all header files
 mkdir include