ci: Check all crate documentation in addition to target-specific ndk (#129)

* ci: Check all crate documentation in addition to target-specific ndk

Only `ndk` docs are built for all `android` targets but the rest of the
documentation remains untested. Especially now that we're expanding to
provide more complete docs for ndk-build and cargo-apk should everything
be tested, to make sure that things like intra-doc links remain in
working order.

* ci: Don't build target-agnostic ndk docs for all targets

The previous commit tests all documentation for all features and crates
(not just `ndk`). The NDK crate and its documentation build without
issues thanks to the guards introduced by the `"test"` feature. Since
the `ndk` docs do not depend on any target-specific flags (and `ndk-sys`
does not contain any documentation) is is fine to build-test it just
once for the host architecture.

Author: MarijnS95

.github/workflows/rust.yml

@@ -73,6 +73,13 @@ jobs:
       run:
         cargo test -p cargo-apk --all-features
 
+    - if: runner.os != 'Windows'
+      name: Document all crates
+      env:
+        RUSTDOCFLAGS: -Dwarnings
+      run:
+        cargo doc --all --all-features
+
     - name: Install cargo-apk
       run:
         cargo install --path cargo-apk
@@ -82,9 +89,3 @@ jobs:
 
     - name: Cargo apk build for target ${{ matrix.rust-target }}
       run: cargo apk build -p ndk-examples --target ${{ matrix.rust-target }} --examples
-
-    - name: Check NDK docs for ${{ matrix.rust-target }}
-      env:
-        RUSTDOCFLAGS: -Dwarnings
-      run:
-        cargo doc -p ndk --target ${{ matrix.rust-target }} --all-features