Specifically install maxtext 0.2.1 in documentation

Author: bvandermoon

docs/build_maxtext.md

@@ -61,7 +61,7 @@ source ${VENV_NAME?}/bin/activate
 
 # Install MaxText with the [runner] extra
 # This enables Docker image building and workload scheduling via XPK
-uv pip install maxtext[runner] --resolution=lowest
+uv pip install maxtext[runner]==0.2.1 --resolution=lowest
 ```
 
 > **Note:** The `maxtext[runner]` extra includes all necessary dependencies for building MaxText Docker images and running workloads through XPK. It automatically installs XPK, so you do not need to install it separately to manage your clusters and workloads.

docs/install_maxtext.md

@@ -40,19 +40,19 @@ source maxtext_venv/bin/activate
 # installation option from this list to fit your use case.
 
 # Option 1: Installing maxtext[tpu]
-uv pip install maxtext[tpu] --resolution=lowest
+uv pip install maxtext[tpu]==0.2.1 --resolution=lowest
 install_maxtext_tpu_github_deps
 
 # Option 2: Installing maxtext[cuda12]
-uv pip install maxtext[cuda12] --resolution=lowest
+uv pip install maxtext[cuda12]==0.2.1 --resolution=lowest
 install_maxtext_cuda12_github_dep
 
 # Option 3: Installing maxtext[tpu-post-train]
-uv pip install maxtext[tpu-post-train] --resolution=lowest
+uv pip install maxtext[tpu-post-train]==0.2.1 --resolution=lowest
 install_maxtext_tpu_post_train_extra_deps
 
 # Option 4: Installing maxtext[runner]
-uv pip install maxtext[runner] --resolution=lowest
+uv pip install maxtext[runner]==0.2.1 --resolution=lowest
 ```
 
 > **Note:** The `install_maxtext_tpu_github_deps`, `install_maxtext_cuda12_github_dep`, and