Add initial version of games catalog

[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 a first version of the Catalog of games:

• Catalog games can be created by adding NFG/EFG files to catalog dir
• pygambit can load games from outside the pygambit code (in catalog dir)
• Moves several game files from contrib/games to catalog to test catalog functionality
• Updates tutorials to load from catalog
• Adds a page to the docs with all Catalog games, linked to on the docs navbar which just has the game slug and title for now
• Adds a script called src/pygambit/update_catalog.py that you run after adding a new EFG/NFG (usage described in developer docs)
• Also contains change from Fix printing normal form games #757

How to review this PR

• Look at the Catalog of games page
• 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

[edwardchalstrey1]

Ed's Todo:

• Move src/pygambit/update_catalog.py to a more appropriate location for a utils script e.g. build_support/catalog
• Add ability for game slug to be a file within a folder e.g. myerson/fig_4_2

[edwardchalstrey1]

@tturocy @rahulsavani this PR can be merged once the game slugs (filenames) and titles for the 4 examples have been updated, i.e. inside the EFG/NFG files in /catalog.

Note that I used the game titles in a few places in test_catalog.py, so will might need to update there too if any are changed.