[REG-1666] Gameplay session docs

[vontell]

Loom video with an overview: https://www.loom.com/share/b6e0ceeaa2e84d3482d65768f6f3e984?sid=ebba2aa5-513a-48db-bbab-6e25957c9e58

Note to reviewers: If you find yourself making a ton of grammatical corrections, I'd prefer if you can just pull down the branch and push those changes directly... I trust your judgement!

This PR introduces documentation for the gameplay session concept. Note that this does not yet cover the web UI side of things yet, and only covers the SDK.

I structured this into three pages:

• An overview
• Getting started and basic usage
• A technical reference (not only for power users, but for ourselves)

---

Find the pull request instructions here

Every reviewer and the owner of the PR should consider these points in their request (feel free to copy this checklist so you can fill it out yourself in the overall PR comment)

• [ ] The code is extensible and backward compatible
• [ ] New public interfaces are extensible and open to backward compatibility in the future
• [ ] If preparing to remove a field in the future (i.e. this PR removes an argument), the argument stays but is no longer functional, and attaches a deprecation warning. A linear task is also created to track this deletion task.
• [ ] Non-critical or potentially modifiable arguments are optional
• [ ] Breaking changes and the approach to handling them have been verified with the team (in the Linear task, design doc, or PR itself)
• [ ] The code is easy to read
• [ ] Unit tests are added for expected and edge cases
• [ ] Integration tests are added for expected and edge cases
• [ ] Functions and classes are documented
• [ ] Migrations for both up and down operations are completed
• [ ] A documentation PR is created and being reviewed for anything in this PR that requires knowledge to use
• [ ] Implications on other dependent code (i.e. sample games and sample bots) is considered, mentioned, and properly handled
• [ ] Style changes and other non-blocking changes are marked as non-blocking from reviewers

[vontell]

@RG-nAmKcAz apologies, for some reason those comments weren't showing up in the code view on GitHub (i think some got outdated from Abby's commit). Going through these now!

[vontell]

@RG-nAmKcAz back to you! I think I got them all, let me know if I am missing something, GitHub is not playing nicely with these comments on this PR...

[vontell]

@RG-nAmKcAz appreciate you committing those last two requests.