Make setting this up with basic CSS convenient

[timabbott]

I noticed https://zulip-archive.rust-lang.org/ doesn't have any CSS; it seems like it'd be worth providing a basic stylesheet (maybe just borrowed from Lean) to provide a nicer feel.

[showell]

This is somewhat related to fixing #12. We don't have the Jekyll layout from Lean, either, so you get a bunch of warnings (plus no real styling):

     Build Warning: Layout 'archive' requested in archive/213224python/47413hello.html does not exist.
     Build Warning: Layout 'archive' requested in archive/213222general/47413hello.html does not exist.
     Build Warning: Layout 'archive' requested in archive/213222general/48863swimmingturtles.html does not exist.
     Build Warning: Layout 'archive' requested in archive/213222general/51687topicdemonstration.html does not exist.
     Build Warning: Layout 'archive' requested in archive/213222general/74282newstreams.html does not exist.
     Build Warning: Layout 'archive' requested in archive/213224python/95106streamevents.html does not exist.
     Build Warning: Layout 'archive' requested in archive/213222general/index.md does not exist.
     Build Warning: Layout 'archive' requested in archive/213224python/index.md does not exist.
     Build Warning: Layout 'archive' requested in archive/index.md does not exist.

This is on my radar to fix, just haven't quite gotten there yet.