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
Ok the way it's referenced doesn't give that impression, otoh I don't think it's a problem to demonstate the best way to apply such rules.

 I'm the maintainer of h5bp nginx :) - that repo is written to be generic/drop-in-anywhere. But for example I don't use url-suffix matches myself.