Update docs with how to run tests & add new tests

[edwardchalstrey1]

Closes #592
Closes #599

This PR:

• Refactors the contributor docs page to make it a bit clearer how to work with Git/GitHub and links to the page for installing Gambit from source
• Simplifies instructions for making docs edits
• Adds instructions on how to run tests & add new tests (we can update this further later on with a more specific process)
• Adds a tests/requirements.txt where requirements for running test suite that are not dependencies of PyGambit itself can be added
• Also adds a section at the bottom of the contributors page explaining how to add people to the contributors page with all-contributors

Link to new testing section: https://gambitproject--598.org.readthedocs.build/en/598/developer.contributing.html#testing-your-changes

[edwardchalstrey1]

@StephenPasteris it came up in conversation that you may be adding new tests, so would be good to get your review on this if anything is unclear

[tturocy]

I'm fairly happy with this as it is - we still have a ways to go on articulating a full structure for the tests as that is still evolving, this will certainly do for now.