Add the Home page, the sidebar and Getting Started
Home says what clsl is, shows the README's opening example with the
values it returns and a definition clsl refuses with its message, and
maps the pages; the sidebar links every page. Getting Started gives the
requirements, the systems, installing with stift, loading clsl/vulkan,
the package shader code is written in, a first compute program run on
the GPU through cl-gfx, and running the tests.
