docs: add tensorflow 2.21 sagemaker training entries (#6378)

Adds docs data YAMLs for the newly released TensorFlow 2.21 SageMaker
training images (CPU and GPU), mirroring the shape of the existing
tensorflow-training/2.19 entries (framework/version/accelerator/python/
cuda/os/platform/ga/eop/public_registry + 4 tags).

Images covered:
  * 2.21.0 CPU py312 amzn2023 SageMaker training
  * 2.21.0 GPU py312 cu129 amzn2023 SageMaker training

This is a manual docs-PR equivalent because the release workflow
step4-docs-pr auto-skipped for tensorflow (not listed in
.github/config/autocurrency-tracker.yml, see
scripts/ci/autocurrency/docs-pr.sh:112). Registering tensorflow in
the tracker is a separate follow-up.

Co-authored-by: Bhanu Teja Goshikonda <bhanugk@amazon.com>

Author: bhanutejagk

docs/src/data/tensorflow-training/2.21-cpu-sagemaker.yml

@@ -0,0 +1,15 @@
+framework: TensorFlow
+version: "2.21"
+accelerator: cpu
+python: py312
+os: amzn2023
+platform: sagemaker
+ga: "2026-07-10"
+eop: "2027-07-10"
+public_registry: true
+
+tags:
+  - "2.21.0-cpu-py312-amzn2023-sagemaker"
+  - "2.21-cpu-py312-amzn2023-sagemaker-v1"
+  - "2.21.0-cpu-py312-sagemaker"
+  - "2.21-cpu-py312-sagemaker"

docs/src/data/tensorflow-training/2.21-gpu-sagemaker.yml

@@ -0,0 +1,16 @@
+framework: TensorFlow
+version: "2.21"
+accelerator: gpu
+python: py312
+cuda: cu129
+os: amzn2023
+platform: sagemaker
+ga: "2026-07-10"
+eop: "2027-07-10"
+public_registry: true
+
+tags:
+  - "2.21.0-gpu-py312-cu129-amzn2023-sagemaker"
+  - "2.21-gpu-py312-cu129-amzn2023-sagemaker-v1"
+  - "2.21.0-gpu-py312-sagemaker"
+  - "2.21-gpu-py312-sagemaker"