formats.kaitai.io: support tags
In #572 we've added support for `meta/tags`, which allows more flexible navigation for formats repo. In #728, we were talking about adding documentation for that.

Now it's time to add that extra navigational support into where it will be most visible and useful — https://formats.kaitai.io.

The proposed design is very straightforward: instead of appearing only in one category, a file will appear in multiple categories — primary, as per directory structure, and secondaries, as per `meta/tags` contents.