Introduce page state management

[n0rdlicht]

Problem

We might want pages in a draft/preview/unpblished state before having them live on the site. Currently this is possible via a "magic" filename prefix of _ (e.g. see #107). However, it would be better to use metadata in the frontmatter to indicate the current publication status, more similar to a CMS.

Proposal

Add a key of status to the frontmatter of any page:

...
# can be public, unlisted or draft
status: public
...