Simplify week notes' directory structure
Instead of having them under /posts/ we can instead store them in their own content type, with a better directory and URL structure. This requires updating any existing links to them on the site, using `ref`, but also so any external links will work, too. We can also remove `tags`, `series` and licensing information as they're now handled by the `week-notes` folder, and remove the `slug`, which no longer makes sense. We add the `type: posts` to make sure that Hugo treats them as articles. Closes #1006.