Fix documentation for CAGRA BBQ build overloads #2961

Summary

Track the existing documentation bug for the half, int8_t, and uint8_t CAGRA BBQ build overloads in a separate documentation PR, as requested by @aamijar during review of PR #2960.

Affected areas

• cpp/include/cuvs/neighbors/cagra.hpp: the three BBQ build overloads use @copydoc targets whose signatures wrap onto a second line.
• fern/pages/cpp_api/cpp-api-neighbors-cagra.md: the review reported stray signature fragments before these overloads and empty dataset parameter descriptions.

The reported fragments occur at Line 1142, Line 1165, and Line 1188 in the reviewed Fern page. Line numbers can change.

Required changes

1. Check how the documentation pipeline resolves the wrapped @copydoc targets.
2. Update all three targets to a form that resolves correctly, such as a complete single-line reference. Alternatively, document each overload directly.
3. Regenerate the Fern page.
4. Check that each overload has the correct scalar type, return documentation, and parameter descriptions.

This fix restores usable API documentation without expanding the scope of the dataset-view pass-by-value change.

Acceptance criteria

• All three overloads have populated descriptions and dataset parameter documentation.
• The generated page contains no stray @copydoc target signature fragments.
• The documented signatures and return types match the corresponding C++ overloads.
• The documentation generation output has no unresolved-reference warnings for these targets.

Context

• PR: Pass dataset views by value instead of const& #2960
• Review thread: Pass dataset views by value instead of const& #2960 (comment)
• Requester: @aamijar