rust: ptr: rename `ProjectIndex::index` to `build_index`

rust: ptr: rename `ProjectIndex::index` to `build_index`

The corresponding `SliceIndex` trait in Rust uses `index` to mean the
panicking variant, which is also being added to `ProjectIndex`. Hence
rename our custom `build_error!` index variant to `build_index`.

Suggested-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/rust-for-linux/DI5LLN2V3XCS.34H4CG99N4MPA@nvidia.com
Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260602-projection-syntax-rework-v2-1-6989470f5440@garyguo.net
[ Reworded docs slightly. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>