Entry page example with links to WAM/RWPM

Hello,

Since we've been talking a lot about entry pages and how various UAs should
handle affordances without a proper example that we can reference, I
decided to build a quick and dirty example.

It's built on top of an existing Web Publication: Why's (Poignant) Guide to
Ruby <https://poignant.guide/>

The code for this example is available at:
https://github.com/HadrienGardeur/webpub-manifest/tree/gh-pages/examples/why

The example itself is available at:
https://hadriengardeur.github.io/webpub-manifest/examples/why/index.html

First of all, there's an entry page with the following things:

   - a link in the header to a WAM using rel=manifest
   - a link in the header to a RWPM using rel=publication
   - a link to the publication itself
   - a link to a basic viewer that can move backward/forward in the default
   reading order

The WAM itself is extended as well:

   - the display member is set to "publication"
   - there's an additional reading_order member

The RWPM is a little more complex since it contains the ToC and a few
additional metadata.

I didn't include the list of resources in these two manifests, to keep the
example as simple as possible.

I might port another example as well, this way we can avoid some of the
cross origin issues.

Hope that this can be helpful,
Hadrien