Fix API reference docs to auto-discover all public modules

[mollyheamazon]

Description:

The API reference docs were manually listing only a handful of modules per subpackage, leaving most user-facing classes (e.g. ModelTrainer, SFTTrainer, ModelBuilder, Pipeline, sagemaker.core.shapes) undocumented on the readthedocs site.

Changes:

• Replaced manual .. automodule:: entries in all 4 API RST files (sagemaker_core, sagemaker_train, sagemaker_serve, sagemaker_mlops) with .. autosummary:: :recursive:
• Sphinx now auto-discovers all public modules, so new modules are documented automatically without any manual RST updates
• Updated conf.py to suppress internal/non-user-facing modules (telemetry, tools, container_drivers, runtime_environment, model_server, detector, validations) and suppress known duplicate cross-reference warnings caused by shared class names across modules
• Added docs/api/generated/ to .gitignore — this directory is generated at build time by ReadTheDocs and should not be committed

Testing: Verified locally with make html — docs build successfully and all previously missing classes are now rendered.

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[mollyheamazon]

Check the new build to see the updated doc: https://sagemaker--5737.org.readthedocs.build/en/5737/

Current v3 documentation is missing many submodules: https://sagemaker.readthedocs.io/en/stable/api/index.html
Such as SourceCode class, which is now updated to be present in the new build: https://sagemaker--5737.org.readthedocs.build/en/5737/api/generated/sagemaker.core.modules.configs.html#sagemaker.core.modules.configs.SourceCode
And previously in v2: https://sagemaker.readthedocs.io/en/v2/api/training/model_trainer.html#sagemaker.modules.configs.SourceCode