svgbob should run after gettext

[jooyunghan]

svgbob translates _text_ into *text*, which makes gettext fail to translate paragraphs with _text_.

[mgeisler]

svgbob translates _text_ into *text*, which makes gettext fail to translate paragraphs with _text_.

Oh, interesting! I did not know about that problem yet...

Our instructions already ask people to run svgbob after gettext — but that was simply done so that translators have the possibility to translate the diagrams.

Now that we know that we only get a warning due to the undefined preprocessor, I'm fine with adding the before/after key to the book.toml.

Going further, we could consider adding the gettext preprocessor to the normal flow — people who want to test the book locally already have to install both mdbook and mdbook-svgbob, so asking them to install i18n-helpers too isn't much of a change. Doing so should simplify the publish.yml script a bit.

[jooyunghan]

This was a regression from #421. Adding before to links caused svgbob running before gettext.

I agree that adding gettext to fix the warning (undefined preprocessor). We can make it a no-op when po-file is not specified.

[jooyunghan]

Can we merge this?