CI: Remove deprecated `actions-rs` and add `cargo doc` test

[MarijnS95]

cargo is bundled in GitHub's standard actions environments.

By running cargo doc we can test for broken intradoc links and other markdown constructs.

[MarijnS95]

error: unresolved link to `NormalMapFormat::R8g8TangentSpaceReconstructedZ`
 --> src/lib.rs:9:53
  |
9 | ...le, while a normal map of format [`NormalMapFormat::R8g8TangentSpaceReconstructedZ`] would still have 3 channels when sampled,
  |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the enum `NormalMapFormat` has no variant or associated item named `R8g8TangentSpaceReconstructedZ`
  |
  = note: `-D rustdoc::broken-intra-doc-links` implied by `-D warnings`
  = help: to override `-D warnings` add `#[allow(rustdoc::broken_intra_doc_links)]`