Add schema checker

Some categorical values could be cleaned up (see TODO comments in the code).

Do we want to restrict additional taxonomies to a fixed set of values? One can list the current values with the following script:

import frontmatter
import pathlib


taxonomies = {}
for filename in pathlib.Path("content/apps").glob("**/*.md"):
    if filename.name == "_index.md":
        continue
    with open(filename) as f:
        doc = frontmatter.loads(f.read())

    for name, value in doc.metadata.get("taxonomies", {}).items():
        if name not in taxonomies:
            taxonomies[name] = set()
        taxonomies[name] |= set(value)

for name, value in taxonomies.items():
    print(f"{name}: {sorted(list(value))}\n")

Merge after #54