Publish game specific struct design documentation

Looking into the new structs, in `spiel_utils.h` it's mentioned that a more explicit struct design exists at `docs/STRUCT_DESIGN.md`, but after some searching I can't seem to find it. Not sure if this is being held back until 2.0, or was supposed to already be out there, but I'd be interested in seeing it.

https://github.com/google-deepmind/open_spiel/blob/fcb98881bffd25a324d5505041c837ca9b7ed0bf/open_spiel/spiel_utils.h#L449-L455

Start with the reference in open_spiel/spiel_utils.h around lines 449-455 and check whether docs/STRUCT_DESIGN.md exists in the repository or history. Confirm what the comment promises and whether the design documentation is intended for the current release. Done means the referenced game-specific struct design documentation is available at the documented location and matches the stated purpose.

Written by the indexing model from the issue text.