Ability to enforce specific docs ordering in kubespray.io

[VannTen]

What would you like to be added

The markdown file are processed by the kubespray.io generation script in lexical order, which is not particularly pertinent.
We should add a way to order the file in the order making the most sense instead.
We simpler way to do that would be to prefix the files (and subfolders of docs, probably) with XXX_, and strip that prefix before rending the index.

Why is this needed

To make our docs site useful. 😆