[DOC] Add extension APIs to runtime API reference

[ymrohit]

Fixes #19348

Summary

• Add extension/module and extension/tensor headers to the Doxygen inputs used by the runtime API reference.
• Expand the module namespace macros so Breathe can resolve the documented extension classes with stable namespace names.
• Add runtime API reference sections for Module, BundledModule, and the tensor extension namespace.

Test plan

• git diff --check origin/main..HEAD
• python -m py_compile docs/source/conf.py
• cd docs && doxygen source/Doxyfile
• Isolated Breathe/Sphinx build of executorch-runtime-api-reference.rst against the generated Doxygen XML
• Verified the rendered runtime API page contains the new Module Extension and Tensor Extension entries

cc @mergennachin @AlannaBurke

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/19385

• 📄 Preview Python docs built from this PR

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⏳ No Failures, 68 Pending, 1 Unclassified Failure

As of commit 0d7aebd with merge base 91aef57 ():
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[ymrohit]

@pytorchbot label "release notes: none"

[GregoryComer]

Looks good. Thanks for the contribution!