Add documentation/ directory and pipeline to build and update documentation

This directory contains a basic skeleton to create the documentation using RestructuredText.

The documentation can be built locally with: make html or make pdf

The second commit adds a pipeline 'pages' to build and update documentation

The documentation will be placed at: https://mupuf.pages.freedesktop.org/valve-infra/

An example of how it'll look can be found at https://ana.pages.freedesktop.org/valve-infra/