We tried using `mkdocs-nav-weight` but it turned out quiet limited. Not
only we have to add specific frontmatter to the Markdown files, but we
could also not hide and sort directories.
This commit introduces awesome-pages, which allows us to customize the
order of pages and not list the content of the "validators" directory in
the left menu.
We used to have those to preserve the order of the pages when generating
the documentation with MkDocs. This commit introduces the
`mkdocs-nav-weight`, that allows us to make that order without having
those prefixes.