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
It's the case for [this repo](https://github.com/igrigorik/istlsfastyet.com) - or have I misunderstood the purpose of this file? I don't see it as a general-purpose example.