Canvis a "Improve the process for contributing to documentation"

The documentation is currently organized on Antora, a tool allowing multi repository doc website in Ascii doc.

The Decidim doc is organized like this:

• Getting started

• Configure

• Admin

• Contribute

• Develop

• Understand

• Release notes

It has several issues, mainly:

1. It mixes different types of documentation in a same place: Developers, administrators, users, researchers, etc.

2. The documentation source is scattered in 2 different places: on decidim/decidim and on decidim/documentation

3. It's hard for non developers to contribute to it, even for small contributions (like fixing a typo)

4. The multilanguage is not easy to use, as it has to go through the Crowdin project (implying the knowledge of another tool)