Dedicated documentation artefact

[seifertm]

Pytest-asyncio's documentation currently lives it's README. Since the README should only contain the most relevant information about a project the current setup requires a balance between adding more documentation and having a bloated README.

The idea of this issue is to create a build process for a separate documentation artefact.

The documentation should be built using Sphinx and the result should be a static HTML website. All required build files should reside in the docs directory. For the purpose of this issue the build can be performed manually without integration into the CI pipeline. As a starting point the documentation artefact may contain the same structure and contents as the README.

The Sphinx tutorial may help.