Add categories and category index pages.

Jekyll will still automatically generate pages and sidebar links for anything with the `page` layout, as before, but now there are also two hard-coded links to the `articles` and `events` categories. The category index pages have their own layout at `_layouts/category_index.html`.

I also changed the list of posts considered for the 'Getting Started' section from `site.posts` (all posts) to `site.categories.articles` (all articles) instead.