Author guide for leveraging metadata collections

[mathewjordan]

Done looks like

• Shows how users can use /api/facets in custom content
• Points out how relative pathing relates to environmental variables

[mathewjordan]

@markpbaggett Might be clear, but I think the idea for this was that this would get plucked out of the the Create Markdown Content guide.

[mathewjordan]

Also, related to this. I think the Docs section just needs a page for API routes. Not sure where to put it. But that is some what related to this:

#18

Beyond the statically generated metadata collections, the API routes (/api/search?q=whatever) only work in non-static instances. But I did want to document that as a feature.