validate that our documentation has HTML metadata #39
Description
we should fail the CI if any document in our docs is missing HTML metadata
documentation: https://github.com/ploomber/contributing/blob/main/documentation/seo.md
we can implement it as part of the pkgmt check command, since that runs in our CI already: https://github.com/ploomber/pkgmt/blob/main/src/pkgmt/cli.py#L56
we're also missing how to include a preview image. ideally we should have a default one because we wont have unique header images for all sections