Add contribution guide, code of conduct, and update templates

This MR adds a contribution guide, a code of conduct, and updates the MR and issue templates.

Can this MR be accepted

• Added contribution guide and code of conduct (#116 (closed))
• Figured out user registration procedure
• Updated templates (#120 (closed))
  • Issue templates
  • MR templates
• Checked that everything renders as desired
• Add a changelog entry
• Bump version number (not necessary)
• History cleaned-up or squash option set
• Pipeline passes without warnings
• Approved by @herdeanu
• Approved by @jeremiastraub

Related issues

Closes #116 (closed), #120 (closed)