Feat: Support categories

Related to #1168

• Clicking on a category in the /icon/{ID} page is not yet supported, this feature needs to be added
• Custom category sorting in /icons/ page (we extract the categories at build time from the metadata.json file)
• Create custom category pages /category/{name}
• Possibly add some kind of "chapters" view on the right side so that you can quickly go to /icons#category-{name} (using a link tag that makes the page auto-scroll to that category, ideally without loading all of the other icons as well, it would be nice to load the page at the # directly)
• Optional: Sort icons by categories in /icons/ page (split them)