Increase number of players beyond 2 for Texas Hold'em

[ryanzhangofficial]

In real life scenarios, games like Texas Hold'em can have up to 6 players at a final table. Robust support for 2+ agents needed across games.

[laminair]

For the time being I have added a note about the two-player limitation in the docs:

arena/docs/games/catalog/texas_hold_em.md

Lines 3 to 5 in 2719d5a

	!!! warning "2-player only"
	Texas Hold'em currently supports heads-up (2-player) play only. Multi-player support is in progress — follow [#41](https://github.com/OutplayArena/arena/issues/41) for updates.