Documentation internationalization — The basics

**Warning:** Do not accept yet, wait for approval in #5899.

• Updated the Makefile to generate the templates in the locale folder.
• Created a folder for the Spanish (es) translation.
• Added necessary fields to conf.py.
• Generated the Transifex configuration file.
• Added *.mo to .gitignore.
This updates the `.pot` files, correct? Can we add this as a step to the release script so that we will automatically "true up" at each release, in case people do forget. I also think we want to keep contributions as simple as possible. Making people install and run some command-line stuff just to fix documentation is a little bit too much of a burden. So we'll accept `.pot` updates with docs fixes, but won't require them.