Internationalize the documentation

[Gallaecio]

I needed the tutorial in Galician for a workshop I’m giving this weekend, so I though I’d take the chance to do this properly.

Changes:

• Short description of the internationalization and localization process.
• Addition of POT files and PO files for Galician, although only the tutorial page is translated.

Notes:

• Having the POT files under version control is common practice, even though they are generated files. More so for PO files, which we could remove from the repository if we ever move to a platform like Transifex, but common practice is to keep them in the same repository as the source code, with the same version tracking. Feel free to ignore their content when reviewing these changes, though.
• For the languages to actually show up in Read The Docs, we need to create separate projects for them. Because of this, and because it is also common practice, we should define some requirements that a translation has to meet before we will consider adding it to Read The Docs. For example, we could require 90% completion of the intro pages and 50% of the topics. My Galician translation, for example, should definitely not warrant it being published in Read The Docs.
• While these changes should be enough for any experienced open source translator to translate the documentation, if we ever want to reach a wider translation audience we should consider Transifex, a kind of GitHub for translations (proprietary, yet popular among open source projects for being free, popular and effective).

Fixes #3511

[codecov]

Codecov Report

Merging #4399 into master will not change coverage.
The diff coverage is n/a.

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