documentation: add dry-run and resource reuse docs to existing RST pages

[amazeAmazing]

Issue #, if available:

Description of changes:

Summary

Adds documentation for two features (dry_run=True and reuse_resources=True) to the existing Sphinx RST pages rather than creating new standalone pages.

Changes

File	What was added
docs/model_customization/model_customization.rst	"Dry-Run Validation" under Key Features — training-focused, with code example and link to evaluation page
docs/model_customization/evaluation.rst	"Dry-Run Validation" section with BenchMarkEvaluator and CustomScorerEvaluator examples
docs/model_customization/deploy_sagemaker_endpoint.rst	"Reuse Deployed Resources" section — source-ID resolution table, ModelBuilder usage examples, default behavior, and limitations
sagemaker-serve/src/sagemaker/serve/bedrock_model_builder.py	Updated docstring: simplified reuse description
v3-examples/.../bedrock-modelbuilder-deployment.ipynb	Shortened reuse intro text (1 line)

Testing

• make html in docs/ builds without errors
• Verified rendered pages at:
  • model_customization/model_customization.html
  • model_customization/evaluation.html
  • model_customization/deploy_sagemaker_endpoint.html

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.