Align documentation with NVIDIA cuML branding and repository migration

[csadorf]

Update the cuML documentation to reflect the NVIDIA GitHub organization migration and the revised NVIDIA cuML branding introduced in #8464.

The documentation is partially aligned: the landing-page title and Sphinx project metadata use NVIDIA cuML, and the version picker uses docs.nvidia.com. Several other documentation surfaces still use the previous product positioning and canonical locations.

Changes

• Position NVIDIA cuML as an open-source CUDA-X Data Science library rather than primarily as “Part of RAPIDS.” RAPIDS can remain as the name of the broader ecosystem.
• Replace the 9 documentation references to github.com/rapidsai/cuml with github.com/NVIDIA/cuml, including the navbar, generated source links, issue links, notebooks, and examples.
• Replace the 10 cuML self-links under docs.rapids.ai/api/cuml with relative documentation links or equivalent docs.nvidia.com/cuml targets.
• Update the remaining cuDF, RMM, and nvForest documentation links to their canonical docs.nvidia.com locations.
• Replace the primary RAPIDS community CTA with the CUDA-X Data Science community link used by the README.

Intentional references to the RAPIDS Release Selector, RAPIDS ecosystem packages, and historic article titles should remain unchanged.

Acceptance criteria

• docs/ contains no references to github.com/rapidsai/cuml.
• docs/ contains no cuML self-links under docs.rapids.ai/api/cuml.
• The landing page uses the NVIDIA cuML and CUDA-X Data Science positioning established in Update README for NVIDIA cuML branding #8464.
• cuDF, RMM, and nvForest cross-documentation links use their canonical NVIDIA documentation homes.
• The documentation build completes without new warnings, and the updated links and intersphinx inventories resolve successfully.

Follow-up to #8464