Librarian utility

[dmundra]

A lot of changes but I have summarized them here:

• Renamed the catalog to match the order of the standards (wcag-2.0 then 508). The catalog is also now auto-generated with a librarian script using values from the catalog/data directory. I copied values from the manually created catalog to generate the data ones.
• Ignoring markdowns in opat directory from prettier because it will keep reformatting the generated version.
• Changes to the various README files, CLI documentation, and also added librarian documentation.
• Regenerated the drupal-9.markdown example with the slight tweaks to the catalog. The catalog now has additions to make it easier to match some of the text expected in the output like 'Applicable Standards/Guidelines' and 'Chapter/Table' differences (update the schema to facilitate that).
• Updated version of js-yaml and added script commands to help generate some of the markdown files without needing to remember the whole command.
• Updated tests and error handling.

Closes: #77

Need follow-up tickets:

• Improve the test coverage for the librarian utility. Add other VPAT catalogs. Added Add other VPAT catalogs to catalog/data directory and update librarian to generate combined ones #105
• Having the output tables link to URL using the alt_id provided. Test those links for accuracy. Added this to Added 'Applicable Standards/Guidelines' and fix table/chapter output including headings #95

[mgifford]

This looks good to me. Thanks Daniel.