Update tutorial 03 to replicate "Stripped down poker" example (#621)

* remove use of poker.efg in test suite

* myerson_2_card_poker.efg -> stripped_down_poker.efg

* added the file tests/test_games/stripped_down_poker.efg

* Fred/Alice -> Alice/Bob; new infoset labels

* update game title in tutorial 3

* change action labels to Bet/Call and Fold

* rename to stripped-down poker

* update references

* match outcome labels to test game

* add full stops

* remove "Betd" typo

* add nodes to same infoset with passing a list of nodes to append_move

* consistent colons

* new name for turorial 03

* minor edits

* typo

* more minor edits

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Co-authored-by: Rahul Savani <rahul.savani@gmail.com>

Author: edwardchalstrey1

doc/pygambit.rst

@@ -23,7 +23,7 @@ They are numbered in the order they should be read.
    tutorials/running_locally
    tutorials/01_quickstart
    tutorials/02_extensive_form
-   tutorials/03_poker
+   tutorials/03_stripped_down_poker
 
 Advanced user tutorials
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