Add GH pages

[bgins]

Summary

Problem

We do not have a mechanism to make the fixtures available to implementations.

Impact

Implementations cannot test without the fixtures.

Solution

Add GH pages that serves from the fixtures directory. It should include an index with a high-level description of the fixtures and links to the fixtures themselves.

The fixtures should be namespaced into directories matching their UCAN version.

Detail

Describe alternatives you've considered

There are options! An implementation could pull in the fixtures as a git submodule or copy-paste directly into their project. Making the fixtures available on endpoints makes them easily accessible.