Add 'free-threaded' to style guide (#1353)

Author: arunkumargururaj07-star

documentation/style-guide.rst

@@ -82,7 +82,10 @@ CPU
 POSIX
    The name assigned to a particular group of standards. This is always
    uppercase.
-
+free-threaded
+    The preferred term for the build mode that makes the Global Interpreter
+    Lock (GIL) optional (per PEP 703). Avoid using "No-GIL" to prevent
+    double-negatives (e.g., "non-no-GIL").
 Python
    The name of our favorite programming language is always capitalized.