use jekyll collections instead of manually ordering pages with numbers

[cpauvert]

It does not seem so convenient to have the index of the page prefixed in the file name (eg. 03-foo.md) especially when there is a need to reorder, as this involves massive renaming.

I'm not completely familiar with the organization of the pages via Jekyll here, but could it be possible to use collections (see "When to use a post, a page, or a collection" section in https://jekyll-one-org.github.io/pages/public/learn/bookshelf/jekyll_collections/)

Could be relevant in the case of recent reshuffling idea #117 or to avoid rechanging all internal links if one page is moved or added.