Publish game specific struct design documentation

[Popret]

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.

open_spiel/open_spiel/spiel_utils.h

Lines 449 to 455 in fcb9888

	// =============================================================================
	// Struct Macros
	// =============================================================================
	// These macros reduce boilerplate when defining game-specific structs.
	// They are placed outside the namespace as they define new types.
	// See docs/STRUCT_DESIGN.md for detailed documentation.

[lanctot]

Whoops! We'll fix that, sorry.

This wasn't done on purpose (e.g. not held back for OpenSpiel 2.0), we probably just forgot to include it in the change. I'll check with the initial controbutor.

But we are still finalizing the struct design so it's under heavy development at the moment. Especially the ActionStruct, you will see a major update coming soon (which we'll highlight in the OpenSpiel 2.0 announcement). I will also try to give an overview in the OpenSpiel 2.0 announcement that should help explain everything.

[lanctot]

(if you're curious about what's new you can check out 4dd2593 and the next 5-6 commits)

[lanctot]

Confirmed that we left it out on purpose because it kept changing. Now thar the struct APIs are mostly decided / stable, we need to rewrite that document.

But might still be a bit before it goes public. We have been working a lot on these lately, so the topic is kind of fresh. It might be best to use this thread to ask any questions you may have.

Tagging @jhtschultz @dhennes who have also been involved in the discussions.