`hide_authors` should be a theme option, not a build-time change

[fwkoch]

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I was looking at the jupyter book workshop template with @pancakereport, and we noticed the project authors were not appearing on individual pages. Looking at the underlying page JSON data, the authors were not there at all. This felt unexpected, as pages always inherit project authors, if they are not defined on the page.

It took us some time to notice the site.options.hide_authors: true flag tucked away in the myst.yml; that was just deleting authors from the pages entirely at build time. (Mind you, this was a feature I implemented 3 years ago, hah...)

Anyway - this feels like a bad pattern. hide_authors should be a theme-level flag (similar to some of the structural options like hide_toc, hide_outline, etc). Then, the authors will still exist in the page metadata, they just will be hidden on the page. We could implement other similar flags, like hide_date or something, that do not impact the metadata but allow more control over the rendered frontmatter block.