Add info about layer mask exporting to physics_introduction.rst (#10242)

* Update physics_introduction.rst

The docs page for raycasting provides a link to the physics introduction page for how to set a collision mask on a raycast created in code. The physics introduction page does not mention export annotations for exporting a collision mask. This pull request adds a brief bit about exporting layer masks, and links to the export annotation docs.

---------

Co-authored-by: tetrapod <[email protected]>

tutorials/physics/physics_introduction.rst

@@ -180,6 +180,12 @@ would be as follows::
     # (2^(1-1)) + (2^(3-1)) + (2^(4-1)) = 1 + 4 + 8 = 13
     pow(2, 1-1) + pow(2, 3-1) + pow(2, 4-1)
 
+Export annotations can be used to export bitmasks in the editor with a user-friendly GUI::
+
+    @export_flags_2d_physics var layers_2d_physics
+
+Additional export annotations are available for render and navigation layers, in both 2D and 3D. See :ref:`doc_gdscript_exports_exporting_bit_flags`.
+
 
 Area2D
 ------

tutorials/scripting/gdscript/gdscript_exports.rst

@@ -298,6 +298,8 @@ It must be noted that even if the script is not being run while in the
 editor, the exported properties are still editable. This can be used
 in conjunction with a :ref:`script in "tool" mode <doc_gdscript_tool_mode>`.
 
+.. _doc_gdscript_exports_exporting_bit_flags:
+
 Exporting bit flags
 -------------------