Add docs for including machine-generated Arabic translations in e2e tests

[zackkrida]

Problem

We currently have an undocumented practice: Whenever we add new text to Openverse requiring translation, which is included in Visual regression tests whose snapshots are updated, we hardcode manual Arabic translations to the https://github.com/WordPress/openverse/blob/main/frontend/test/locales/ar.json file. This allows us to approximate how RTL and Arabic support looks in the actual site.

You can see this process in action on this PR.

Description

Add docs explaining how this works, and instructing folks to go to translate.google.com to get a machine translation, to https://docs.openverse.org/frontend/guides/translate.html.

[obulat]

#3725 has added a script for getting the Arabic translation automatically, and also added documentation for it: https://docs.openverse.org/frontend/reference/i18n.html#test-locales