Add Catalog of games

[edwardchalstrey1]

Issues closed by this PR

• Closes Create initial games catalog from contrib/games #731

Description of the changes in this PR

This PR adds infrastructure and documentation for an a first version of the Catalog of games:

• Moves all the game files from contrib/games to src/pygambit/catalog_game_files
• Catalog games can be defined in code in src/pygambit/catalog_games.py or from file in src/pygambit/catalog.yml
• Adds automatically generated API reference docs for all Catalog games
• Updates all tutorials that read from EFG/NFG files to instead load from the Catalog
• Replaces the "Sample games" header on the docs navbar with a link to the list of Catalog games in the API docs
• Adds a script called src/pygambit/catalog_update.py (usage described in developer docs)

How to review this PR

• Look at the Catalog games list to see what we have now
• Review the developer docs page on Updating the Games Catalog
• Take a look at the Loading games from the catalog section added to Tutorial 1

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[edwardchalstrey1]

@tturocy @rahulsavani This is now ready for review - there's a lot happening in this PR, so please take a look at the docs links I've added in the top comment of this PR which should explain how everything now works.

I'm going to create some additional feature issues for the catalog and add them to the list in #394

After your review, we can make a bunch of PRs to update the catalog entries for these purposes:

1. To update the names and any metadata for all the games in catalog.yml (and either remove or comment out any that aren't wanted) (i.e. following the "Add game files to the catalog" guide from the Updating the games catalog page)
2. To move games used in tests into the catalog_games.py (using the "Code games for the catalog" guide) and update the tests to load those games from the catalog

We should decide whether we want to merge this branch into master now and make those PRs there, or alternatively you could branch off enhancement/731 and make PRs that target this branch.