Prefer matchCallbacks to RegExpRoutes in the docs

[jeffposnick]

Pretty much universally throughout our docs and samples, we end up using RegExps for routing.

In retrospect, this ends up being fairly confusing and error prone, especially since we have to explain the nuances around cross-origin RegExp routing.

We should audit the current examples in the docs and when it makes sense to switch to a simple matchCallback, do so.

E.g.

registerRoute(
  /^https:\/\/fonts\.googleapis\.com/,
  new StaleWhileRevalidate({
    cacheName: 'google-fonts-stylesheets',
  })
);

makes much more sense as

registerRoute(
  ({url}) => url.origin === 'https://fonts.googleapis.com',
  new StaleWhileRevalidate({
    cacheName: 'google-fonts-stylesheets',
  })
);

[jeffposnick]

Fixed by google/WebFundamentals#8680