Reorganize the `About Mithril` section in the documentation website

[jpraynaud]

Why

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

What

Implement the new About Mithril section structure in the documentation website.

How

• Implement About Mithril section new structure
• Rotate next version to current
• Handle redirection from previous URLs to new ones