Translate docs

[JrooTJunior]

It would be nice to have the docs translated to different languages to make easy dive into aiogram and bot development, it's possible with sphinx and ReadTheDocs native using I18n and this instruments is already used in this project.

In due to 2.x branch of development soon will be finished and only 3.x will be supported (after public release) we don't need to translate 2.x docs, so, the only new docs should be translated.

Goals

• [ ] Configure Sphinx and ReadTheDocs project to use I18n
• [ ] Describe how to translate the docs in contribution guide
• [ ] Translate texts to different languages:
  • [ ] Ukrainian (by @JrooTJunior)
  • [ ] Portuguese (by maybe you)
  • [ ] ... (you can ask to add new language to this list if you can make translation to this language)

Related docs

• Sphinx I18n: https://www.sphinx-doc.org/en/master/usage/advanced/intl.html
• ReadTheDocs Internationalization: https://docs.readthedocs.io/en/stable/localization.html