docs(i18n): internationalized the documentation

• Updated the Makefile to generate the templates in the locale folder.
• Added necessary fields to conf.py.
• Updated the Transifex configuration file to cover the documentation.
• Added *.mo and .po to .gitignore.
• Updated the documentation about writing documentation and releases.

Fixes #5899
Merge pull request #6952 from Gallaecio/doc-i18n

docs(i18n): Documentation internationalization — The basics