First, did the Gemfile stuff from the official GitHub instructions:
https://help.github.com/articles/setting-up-your-github-pages-site-locally-with-jekyll/#step-2-install-jekyll-using-bundler

Then, rather than `jekyll new .` like those instructions recommend,
which creates a whole scaffolded directory structure and files and stuff
that we don't want, instead copied the minimal set of files (well, the
entire _sass/ folder, but other than that, the minimal set of files)
from [johnotander/pixyll] to make

    jekyll serve && open http://localhost:4000/see-pixyll-in-action.html

work and display with the right styles/the Pixyll theme.

[johnotander/pixyll]: johnotander/pixyll@dca7f12

Gets rid of this message during `jekyll serve`:

        Pagination: Pagination is enabled, but I couldn't find an index.html page to use as the pagination template. Skipping pagination.

Based on https://jekyllrb.com/docs/configuration/#default-configuration
except for Sass, which isn't documented there, but is documented at
https://jekyllrb.com/docs/assets/#sassscss

Except for _posts/, which isn't configurable, but it turns out we don't
need it at all!

Note that at first I tried setting `source: _jekyll`, but then Jekyll
couldn't find index.html, so I tried to symlink _jekyll/index.html to
../index.html, but it turns out that for security reasons Jekyll only
supports symlinks within site.source:
jekyll/jekyll#4710

TO BE CONTINUED