Add nashpy to the list

[drvinceknight]

Link to project

https://nashpy.readthedocs.io/en/stable/

Describe your suggesiton

The docs are written using the diataxis framework and I feel offer two cool features:

• The reference documentation is essentially as good as a standalone Game Theory text book (it is the course text for a 3rd year University course)
• There is an entire section for contributors which itself is written using the diataxis framework (this is not complete) -- I often use this to onboard research students to software development.

:)