Add GH issue tutorial request template and tutorial submission guide

[edwardchalstrey1]

This PR adds a workflow for adding tutorials to the documentation, see the updated docs editing section of the contributor guide here which now contains a section specifically on adding Jupyter notebook tutorials.

• Add new issue template for tutorials
• Add a "How to add tutorials" section to the contributor docs which explains:
  1. Open an issue with your tutorial request
  2. If you seek to author the tutorial yourself, open a new PR that:
     • Adds the new tutorial as a .ipynb in doc/tutorials in the appropriate sub-folder (or creates a new one)
     • Updates doc/pygambit.rst to ensure the tutorial is listed in the docs at an appropriate location