DOP-5303: Handle ast file facets in postprocessor

[rayangler]

Ticket

DOP-5303

Notes

• There was an additional change that needed to be done to allow facets to be propagated properly to ast files.

To test:

1. Go to this staging link
2. Open up dev tools and go to the network tab.
3. Look for page-data.json for the current staged page. Navigate to result.data.page.facets and the repo-wide facets should now be there, along with the new "reference" genre facet.

README updates

• • This PR introduces changes that should be reflected in the README.md and/or HACKING.md, and I have made those updates.
• • This PR does not introduce changes that should be reflected in the README.md and/or HACKING.md

[mmeigs]

Great!