CSS assets don't reflect swift-doc's versioning

[Lukas-Stuehrk]

Currently, swift-doc generates the CSS assets by simply downloading the latest version of the minified CSS file from GitHub. This will break old versions once we keep iterating on the CSS and introduce backwards-incompatible changes.

Therefore we should find a way to use the correct version of the CSS files that relates to the version of swift-doc which is used to generate the documentation.

I think the easiest solution is to switch back to get the CSS from the package resources. There was already an attempt in #192. Unfortunately this introduced a lot of issues and needed to be reverted. In the meantime, the minimal Swift version for swift-doc was bumped to Swift 5.3, so from my understanding this approach should be safe now.