docs: clarify NCore positioning and NuRec relationship

[aperec-nvidia]

What changed

• Reworked the README opening to present NCore as a general, self-contained multi-sensor data platform.
• Kept NVIDIA Omniverse NuRec as one downstream use rather than NCore's exclusive purpose.
• Incorporates Janick's feedback from the source sheet.

Repository metadata to apply after review

• About: NVIDIA NCore is an open, self-contained data platform with a focus on robotics and autonomous vehicle data, providing a consistent multi-sensor representation, APIs, and simulation tools.
• Topics: nvidia, physical-ai, autonomous-vehicles, robotics, neural-reconstruction, nurec, simulation, data-format, multi-sensor, lidar, sensor-simulation, dataset, openusd

GitHub About text and topics are repository settings, so they are listed here for maintainer review rather than changed in this branch.

Validation

• git diff --check
• README-only change

Source copy: https://docs.google.com/spreadsheets/d/1IxLuj3hxhN5y3d0fmIxGh9n2LiSq2Su2cTe9qpdAOKI/edit?gid=823075834#gid=823075834

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[janickm]

Looks good to me, thanks @aperec-nvidia - can you please make sure to put these changes into a single commit (currently it's two)?

[janickm]

/ok to test 16ac68b