feat: compress css

[pfitzseb]

Generated with Sass.compile_file("scss/documenter-light.scss", "themes/documenter-light.css"; output_style=Sass.compressed)

assets/html/themes on master
❯ ll
total 768K
-rw-r--r-- 1 pfitzseb pfitzseb 450K Jun 28 12:10 documenter-dark.css
-rw-r--r-- 1 pfitzseb pfitzseb 313K Jun 28 12:10 documenter-light.css

assets/html/themes on sp/compress-css
❯ ll 
total 644K
-rw-r--r-- 1 pfitzseb pfitzseb 381K Jun 28 12:08 documenter-dark.css
-rw-r--r-- 1 pfitzseb pfitzseb 258K Jun 28 12:09 documenter-light.css

Doesn't save too much space, but ~20% is not nothing.

Fixes #2153.

[mortenpi]

We should make this the default in DocumenterTools. That would also fix the CI.

[mortenpi]

Thanks!