[DOC] Convert docs from Sphinx to Fern

[cjnolet]

We need to migrate cuVS docs to docs.nvidia.com. NVIDIA is adopting Fern more broadly, so we need to migrate the docs to Fern.

[cjnolet]

Current blocker is Doxygen for the C & C++ APIs. Attempted migration at #2059, #2056, and #2055 without success.