Djm/pkgdown prettier

[dajmcdon]

Improve the pkgdown site (nice and red and matches epipredict).

Checks fail until #377 is merged.

[dajmcdon]

@brookslogan needs ds/clean to pass checks (at least).

[brookslogan]

I think we're still trying to decide on branch organization --- it appears that pkgdown lets you build main & dev documentation sites independently of how you organize your branches (instead basing it on whether you use a 3-number or 4-number version tag) --- but I believe we decided to try to standardize the appearance across both main & dev sites rather than maintain separate styles. So I will try to merge this in. One thing we need to check is whether there is some auto-differentiation of style by pkgdown using red text for dev version numbers that may not be visible against a red bar at the top.

[brookslogan]

Looks good, but before approving & merging, checking about provenance. (Certain themes are exclusive to tidyverse or some other universe/org.)

[brookslogan]

Provenance: epipredict styling copied here was by @dajmcdon following https://pkgdown.r-lib.org/articles/customise.html.

[brookslogan]

Because dev currently has a 4-digit version number, merging this will likely only update the dev site... which appears not to have been built successfully yet; the expected landing page gives a 404. Maybe merging will give it a chance to build with the epidatr sync [& ds/clean] PR[s].

[brookslogan]

Oh, dev didn't exist because #383 wasn't finished yet. No wonder there were no conflicts.