Use a style file when generating HTML documentation.

* doc/Makefile (slime.html): Use --css-include=slime.css

* doc/slime.css: New file. Simple stylesheet adapted from the one
used by Maxima (http://maxima.sourceforge.net/).
Closes #5: website and documentation out of date

* README.md: Add link to manual in github pages (hosted under
capitaomorte.github.io for now).