Add support for reStructured Text markup language

Support for reStructured Text only needs the python `docutils` package to be installed.
This PR adds the installation instruction to the install guide and also to the update guide. Because GitLab uses the system python, installation via packet manager is least painful approach to support for `docutils`

With this PR all `*.rst` will be rendered as reStructuredText markup language.
This implements feature request http://feedback.gitlab.com/forums/176466-general/suggestions/3868963-add-support-for-restructured-text

/cc @senny as discussed.