Reorganize the user manual in the documentation website

[jpraynaud]

Why

In order to make the user manual of the documentation website more clear and easier to navigate through, we want to reorganize its structure.

What

Implement the new user manual structure in the documentation website.

How

• [x] Implement new user manual structure
• [x] Rotate next version to current
• [x] Handle redirection from previous URLs to new ones