Add a new "direct" biblio display style

[brycelelbach]

Add a new biblio display style, "direct", which displays references by their shortname and links to the referenced document. Closes #2032.

[tabatkins]

The feature looks good. Only problem is the busted indents in the docs, which'll break the formatting when I actually process them.

When you fix those, would you mind running Bikeshed over it and including the results in the PR, too?

[brycelelbach]

D'oh. Looks like I missed the fact that the docs use tabs! I was wondering if I should regenerate them.

I've fixed the indents and added regenerated docs to the PR. I also rebased on main (and regenerated the tests and docs afterwards just to be safe).

[brycelelbach]

Thanks!