Update contributing guidelines for issues and the pypi release process
I realized we do not document the PyPI release process in our contributing guidelines so the primary purpose of this PR is to provide that information.
Whilst doing so I've also revised the references to opening "issues" to first link to the GitHub issues page. It is not possible for non-DOI users to open issues in the GitLab repository (although they are readable for everyone). So guidance in the contributing documentation has been updated to note that issues can be opened in GitHub.
Some minor clarification about the continuous integration pipelines is also updated here (the specific tools used to do code and doc-string style checking).
@lstanish since this is documentation I'm tagging you, but if you don't want to review this please ping someone else on the team - thanks.