Minify assets

[carrythebanner]

• Uses Hugo's asset pipeline to minify our custom CSS and Javascript. As a result most CSS and JS moved from the theme's /static directory to /assets.
• Third party libraries stay in /static, but path changed from /static/js/libs to /static/lib. Singular lib is more common, e.g. FHS, and the libraries were a mix of JS and CSS so the js subfolder was a bit misleading.
• Simplified how the theme CSS file was chosen. There was previously some logic in the site's config file to allow for choosing a different theme CSS file, but we aren't using that. Removed that logic and renamed the theme's primary stylesheet from style.default.css to theme.css.
• Updated some dev docs

Fixes #397.

[ionous]

i moved my existing "site/public" folder aside, applied the docker-compose fix for hugo, ran ./shift up, and saw it built successfully.

i compared the new files with the old, and saw only the expected changes. i browsed locally and created an event, all the pages look good.

both locally, and in the deploy preview https://deploy-preview-775--shift-docs.netlify.app/ i can see the .min versions of the assets come down. ( i notice it looks like netlify uses brotli compression on them as well, but maybe only if they are above some threshold )

( i think the site has too many files named 'main' )