invalid applies_to keys can be set in front matter

[shainaraskas]

Describe the bug

I passed the following key:

applies_to:
  deployment:
      serverless: all
      hosted: all

hosted and serverless is not a valid subkey of deployment, so the little tag didn't appear in my page. however, the PR passed CI with no problems

Expected behavior

invalid keys throw CI errors

I realize that we want to allow custom keys, but in the other well-defined errors we should do some validation

Steps to reproduce

applies_to:
  deployment:
      serverless: all
      hosted: all

Tooling

• docs-builder
• migration tooling
• I'm not sure