Refresh PyGambit user guide and add interactive Jupyter notebook option

[edwardchalstrey1]

Changes

• Restructures the PyGambit user guide into a set of notebook tutorials, which are rendered in the docs but can also be run locally by cloning the gambit repo
• Tutorial content is adapted from the user guide but takes a sequential approach to introducing the PyGambit API functionality. The first 3 introductory tutorials should be understandable to someone without game theory knowledge or experience with Gambit, as is explained in the docs
• Closes [Documentation]: Idea: Interactive version of PyGambit user guide/examples #542 - following this format it should be simple to add new tutorials going forward

Reviewers

• Please review PyGambit docs for this PR build compared with current release
• I would suggest someone has a full read through of all the tutorials: all of the sections of the original documentation should still be included as part of one tutorial or another, with the exception of "Lifetime of a game object and its elements" which I removed since I felt it was just explaining a concept in Python, not a specific PyGambit feature - happy to restore it if it feels important!

TODO

• I need to make it clearer that in these notebooks what I did was overwrite node labels with action labels, to make it possible to index Node.children by action label, something which the API does not currently support - see Ted's comment here: Ensure append_move and other functions to apply labels to nodes by default #553 (comment)
  • So it may be worth implementing a solution for Ensure append_move and other functions to apply labels to nodes by default #553 in advance of merging this PR, or alternatively, revert these examples to indexing Node.children by node indices for now
  • One option for now could be reference nodes according to Node.prior_action.label instead of Node.label Edit: For now I have explicitly set Node.label to Node.prior_action.label in the Tutorial code