site: rework guides filters and tags taxonomy

- Rework the filtering system for guides to drop the use of the
  "products", "subjects", and "levels" taxonomies in favor of "tags" and
  "languages"
- This change means that the existing taxonomy functionality integrates
  better with the guides filtering, and there are fewer parameters to
  keep in mind when adding metadata to a guide
  - Only two taxonomies instead of three
  - Only one of those taxonomies are guides-specific (languages)
  - The other taxonomy (tags) works for all content
- Updates how tags and tag pages are rendered in general

Signed-off-by: David Karlsson <[email protected]>

Author: dvdksn