🚀 Feature

We should validate docs/blog/pages frontmatter against a Joi schema:

• prevent the user to use bad frontmatter
• display friendly error messages when this happens
• allow unknown frontmatter (user can assign additional metadata to each doc)
• unit-test this validation logic
• try to use the frontmatter TS types in the theme?

Some initial work has been done in #4590 for Docs and Blog, but is not exhaustive as it was not really the point of this PR.

Asking for help from the community to get this work completed in a more exhaustive and polished way.

Please claim this issue if you are willing to help.