Area for samples on amp.dev

### Background: We want to link to sample mini-sites on amp.dev!

It would be really nice to have this up in time for I/O next week, since I'm doing a talk there on a new sample - AMP CAMP, the e-commerce demo site.

We've already created an area in the `amp.dev` domain namespace. From now on, samples can be hosted on App Engine and pointed to `*.samples.amp.dev`. The AMP CAMP site will live at `camp.samples.amp.dev`.

A couple of sample sites are already beautifully showcased at [about/use-cases](about/use-cases). But we now have [several samples on github])(https://github.com/ampproject/samples), and this showcase doesn't contain an index.

I think we really need a landing page that features a box for each sample. That box would contain a screenshot, a link to view the mini-site, a link to find the code, and (in a very few cases) an area to describe any guides we've written.

@sebastianbenz points out that we might want to have an area where nondevelopers can easily view these sites, like the current Use Cases section, and an area where programmers can learn how they were made. Hmm.

One option would be to includes the samples in the current [Examples](https://amp.dev/documentation/examples/) area. This area could have three tiers of samples, in ascending order of complexity
1. samples that illustrate the use of an individual component
1. samples that show techniques
1. samples that consist of full webpages or sites

This is possible, but I think it would take reworking this page to make it simpler, as it's already quite full.

I personally like the idea of a new Samples landing page, at least for a short-term solution. We could create a new menu item under "Documentation". This could potentially even be done in time for I/O!

What do you all think?

/cc @matthiasrohmer @sebastianbenz @pbakaus @CrystalOnScript @demianrenzulli @antoinebisch