Remove unused files, update contributing guide and PR template

[michaelosthege]

Motivation for these changes

This is mostly motivated by the RP template being very opinionated and uncompromising.

Implementation details

I defused the PR template and aligned it more with the one over in PyMC.

I kept the links since they are helpful, but rephrased it such that contributors aren't expected to have a degree in git.

Similarly, I aligned the CONTRIBUTING.md with the PyMC one, keeping some good parts about the use of discussions and issues, but removing the section about development and governance style.

In addition, I removed two files that are no longer used/maintained.

Checklist

• Explain motivation and implementation 👆
• Make sure that the pre-commit linting/style checks pass.
• Link relevant issues, preferably in nice commit messages. No issues present - was discussed offline.
• The commits correspond to relevant logical changes. Note that if they don't, we will rewrite/rebase/squash the git history before merging.
• Are the changes covered by tests and docstrings? No code changes.
• Fill out the short summary sections 👇

Major / Breaking Changes

• None

Bugfixes / New features

• None

Docs / Maintenance

• Contributing guidelines and PR template were updated to align with the ones of the PyMC project.
• Unused Makefile and .github/FUNDING.yml were removed.