Here, the pkgdown docs say that I can supply my own templates (footer, etc) using the templates_path key in _pkgdown.yml. I am struggling to make pkgdown notice my custom footer from https://github.com/wlandau-lilly/drake/commit/2ca610f932be8d504560027b70106b5d57e3f0ea, and I have not been able to find any guiding examples.
So apparently, this feature was not officially implemented: https://github.com/r-lib/pkgdown/issues/486.