Update to mdbook-i18n-helpers 0.3.0 (#1460)

This version has much improved support for the translation of code
blocks. See google/mdbook-i18n-helpers#95 for
details.

Most PO files won’t need any update since most of them don’t translate
the comments in the code blocks. Those that do, can run
`mdbook-i18n-normalize` when they feel like it.

.github/workflows/install-mdbook/action.yml

@@ -16,7 +16,7 @@ runs:
       shell: bash
 
     - name: Install mdbook-i18n-helpers
-      run: cargo install mdbook-i18n-helpers --locked --version 0.2.4
+      run: cargo install mdbook-i18n-helpers --locked --version 0.3.0
       shell: bash
 
     - name: Install mdbook-exerciser