Set-up a translation infrastructure

[ruphy]

We need a simple way to manage translations and evolve the standard with automated consistency checks.

The master copy needs to be developed in English, translations are handled with a separate workflow and can be done separately. With a build-like process translated versions are created and uploaded, without the need for manual merges.

[ruphy]

@bfabio and I decided to try out the following approach:

• use Gettext as underlying backend for translators
• implement a custom .pot/.po utility for parsing our dialect of rst correctly (e.g. understanding the "Key: key" syntax)
• implement two scripts to: generate_pot_merge_po (to create pot files and merge them into existing po files); generate_doc (to generate doc from the master english copy and the translated po files)
• (later) integrate this system with a CI toolchain

[bfabio]

Update: after further discussion, we agreed that a solution based on the built-in i18n mechanism in Sphinx would be more beneficial, resulting in a simplification and a better alignment with upstream practices.

[bfabio]

We kept it simple, see #77.