Align contributor documentation and structure ahead of ECS.

[BrickPi]

• Creates a more coherent contributor quickstart guide
• Ports code of conduct over from the Discord
• Reorganizes floating docs into relevant folders
• Renames the nested README in docs to table-of-contents.md
• Introducing state merging docs without disputed section, resolves Document state merging #306

First part of a broader documentation bringup.
Full resolution of #499 blocked by dataloading rework, buildsystem changes, ECS introduction, and project tasking rework.