Documentation internationalization — The basics

 * [x] Updated the Transifex configuration file.
 * [x] Added `*.mo` to `.gitignore`.
 * [ ] Add `.pot` file regeneration to the `chore(i18n)` part of the release script
   * [x] Added necessary fields to `conf.py`.
   * [x] Updated the Makefile to generate the `.pot` templates in the locale folder.
   * [ ] Update `contribute/release.rst` docs to reflect the new requirements (`sphinx-intl`)
 * [ ] Approval from 3 @Elgg/owners (@ewinslow, TWO, THREE)
 * [x] No one from @Elgg/owners is blocking

Done.

Thanks to your comment I also realized that the ´sphinx-intl update´ is not necessary (it merges the POTs with the POs, which happens already at Transifex.

I’ve also updated the release documentation adding Sphinx to the list of dependencies.