validate that our documentation has HTML metadata

[edublancas]

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

[edublancas]

note that in jupyter-book 0.14 the format has changed from:

html_meta:
    description lang=en: Create line, bar and pie charts from SQL queries in a Jupyter notebook using JupySQL
    keywords: jupyter, sql, jupysql, plotting, matplotlib
    property=og:locale: en_US

to

myst:
  html_meta:
    description lang=en: Create line, bar and pie charts from SQL queries in a Jupyter notebook using JupySQL
    keywords: jupyter, sql, jupysql, plotting, matplotlib
    property=og:locale: en_US