Update usages of obsolete rad2deg() and deg2rad() in docs

There were 4 pages documentation pages still using deg2rad(),
which have been substituted by deg_to_rad() and the examples
tested against 4.0.X.

There was 1 usage of C# Deg2Rad(), now DegToRad().

Some dependent comments and output have been updated accordingly:
(deg_to_rad() is implemented in the Math core functions, not in
the Expressions class), plus some float precission fixes, at
least on my architecture.

Only one mention of rad2deg() was found in all docs.

Fixes #7247

getting_started/first_2d_game/05.the_main_game_scene.rst

@@ -392,8 +392,8 @@ Note that a new instance must be added to the scene using ``add_child()``.
                not degrees. Pi represents a half turn in radians, about
                ``3.1415`` (there is also ``TAU`` which is equal to ``2 * PI``).
                If you're more comfortable working with degrees, you'll need to
-               use the ``deg2rad()`` and ``rad2deg()`` functions to convert
-               between the two.
+               use the ``deg_to_rad()`` and ``rad_to_deg()`` functions to
+               convert between the two.
 
 Testing the scene
 ~~~~~~~~~~~~~~~~~