Add Gambit/OpenSpiel interoperability tutorial

[edwardchalstrey1]

Closes #555 Includes changes from #621

See the tutorial here

and where it fits on the PyGambit docs pages here

Changes

• Adds OpenSpiel tutorial to a new "Interoperability tutorials" section
• Updates local running tutorial instructions to include OpenSpiel
• Also adds a few items to .gitignore

[edwardchalstrey1]

Ok @rahulsavani this is ready to review: https://gambitproject--573.org.readthedocs.build/en/573/tutorials/interoperability_tutorials/openspiel.html

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[edwardchalstrey1]

Thanks @rahulsavani these changes should now be made, you can see in the comment above that "ReviewNB" is now available. The OpenSpiel notebook doesn't show a diff since it's new, but you can see how it works by looking at the small change I applied to the other notebooks (hyphenating to "extensive-form" and "normal-form") which shows a clear diff which should make reviews for updates to tutorials easier going forward.

[rahulsavani]

Thanks @edwardchalstrey1, ReviewNB indeed seems nice and convenient.