docs: list supported point cloud formats

[SKSavale]

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

Issue #7598 requests an explicit list of supported formats for the format argument in the I/O documentation. This change makes the supported point cloud formats easier to discover without requiring users to inspect the source code.

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

Updated the I/O documentation to explicitly list the supported point cloud formats for the format argument:

• xyz
• xyzn
• xyzrgb
• ply
• pcd
• pts

Closes #7598

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.