docs(i18n): Documentation internationalization — The basics

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