Simplify two‐step build

The two‐step build originally installed to `$(BUILDDIR)/data`, but this
installation step is unnecessary (it just copies over files from the
`public/` folder of the first build). This commit uses the `public/`
folder of the first build directly, allowing both builds to have the
same `$(DESTDIR)` which is only used when actually `make install`ing.

This also enables renaming the folders for the first and second build
from `stage1/` and `stage2/` to `data/` and `site/`, which are more
informative.