Add Doppler ICP Registration Documentation

[heethesh]

Type

• Bug fix (non-breaking change which fixes an issue): Fixes #
• New feature (non-breaking change which adds functionality). Resolves #
• Breaking change (fix or feature that would cause existing functionality to not work as expected) Resolves #

Motivation and Context

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is
    updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test
    results (e.g. screenshots or numbers) here.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

• Adds documentation for Doppler ICP registration to the tutorial notebook, including algorithm explanation, parameter details, and example code comparing Doppler ICP with point-to-plane ICP.
• Updated reference.rst with Hexsel2022 citation.
• Fixed typo: goemetric_kernel to geometric_kernel in Python binding, example script, and notebook.

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[heethesh]

@ssheorey this PR is ready for a review