Use simpler location blocks

there's no need for a regex match for the url `/index.html`

Use prefix-matching for assets as that's the nginx-way® and the rule is simpler.

Note that this isn't the case on istlsfastyet.com:

> make sure you are using cache busting filenames or query params
The snippet here applies to this site, but I'm also linking to it as a generic example. Hence, I think it makes sense to keep the regex as is (which, btw, is what HTML5 boilerplate recommends also).