Add documentation build in CI

Build inner code document only for rusty_vault crate and rvault binary,
so option --no-deps is passed to cargo doc command.

.github/workflows/rust.yml

@@ -35,6 +35,8 @@ jobs:
       run: cargo build --verbose
     - name: Run tests
       run: cargo test --verbose
+    - name: Build crate doc
+      run: cargo doc --no-deps
 
 
   windows-test:
@@ -76,3 +78,5 @@ jobs:
       run: cargo build --verbose
     - name: Run tests
       run: cargo test --verbose
+    - name: Build crate doc
+      run: cargo doc --no-deps