Large site now causes long build times

[lawndoc]

With the addition of thousands of pages of documentation in #23, it now takes >40min to build or republish the Jekyll site.

To improve build time, I'm going to create an alternative test config that excludes large directories from the build.

We won't be able to change the site republish time unless we change the setup from GitHub pages and self-host with --incremental.

[lawndoc]

I opened jekyll/jekyll#9434 in jekyll asking for a lazy build feature. It would speed up serve time massively if we could skip the initial build, and then build pages as they are requested.