Update docs about translations

[stsewd]

The docs here needs to be updated https://docs.readthedocs.io/en/latest/localization.html according the recent changes on translations (only can add translations where the user is owner)

[stsewd]

Reading the current docs, seems ok to me, because there is no mention of adding a project from other user, or nested translations allowed, so, I think with the current UI explains the limitations by itself.

[RichardLitt]

Might be worth adding notes so that #3348 and #4152 can be resolved more gracefully in the future.

[agjohnson]

Agreed. We need much better docs around this all. I'm really looking forward to making translations a priority on RTD. I think we can do much better at offering first class support for translations. They are obviously more of an after thought currently.

First steps here I think are:

• Describe how to manage translations better on RTD projects
• Describe patterns for this (how to support multiple translations in one repo)
• Explain the sphinx steps better

Next steps on our side are things like Transifex integration, Weblate integration, and automating the Sphinx configuration for multiple languages.