Build translations from mdbook-i18-helpers

[sakex]

Supports mdbook build and mdbook serve. I'd recommend waiting a bit before submitting and let translators try it by manually patching this PR.

It uses google/mdbook-i18n-helpers#84

[randomPoison]

I don't think this is doing the same thing that the original logic was doing. It looks like the original was putting the archive at book/$po_lang/html/comprehensive-rust-exercises.zip, but with the version you have here they would end up at book/html/$po_lang/comprehensive-rust-exercises.zip. Is this change intentional?

[max-heller]

Likely relevant to this is that the publish workflow now passes additional configuration for each language to a mdbook backend (mdbook-pandoc) to render PDFs. We should be sure this maintains the same functionality:

comprehensive-rust/.github/workflows/build.sh

Lines 33 to 42 in 3afdabf

	# Include language-specific Pandoc configuration
	if [ -f ".github/pandoc/$book_lang.yaml" ]; then
	export MDBOOK_OUTPUT__PANDOC__PROFILE__PDF__DEFAULTS=".github/pandoc/$book_lang.yaml"
	fi
	fi
	mdbook build -d "$dest_dir"
	if [ -f "$dest_dir/pandoc/pdf/comprehensive-rust.pdf" ]; then
	mv "$dest_dir/pandoc/pdf/comprehensive-rust.pdf" "$dest_dir/html/"
	fi