Design update to main index and article index

[kdaily]

This updates the main index page as well as the article index. It 'simplifies' the building of the page to get all categories automatically - no need to add a separate section and code. Now you would add a markdown file to the _categories directory that describes a new category and use the same category name in documentation articles.

This also updates the 'boxes' at the top to be the same as the categories - before, they were hand-curated as well.

This is a big change that warrants discussion, as it changes how content is contributed (I think impacts maintainers more, but would appreciate feedback on potential user or docs creator impact).

[Aryllen]

This is great! The two "solutions" I found while I was looking for fixes to #677 took a similar route. I was trying to code up the one that didn't involve keeping a file per category to decrease maintenance and number of files. However, I like this way better; it gives a paper trail for how to make the documentation and the maintenance is low.