docs: add read only data handling to programming rules

Signed-off-by: Ronald Caesar <github43132@proton.me>

docs/PROGRAMMING_RULES.md

@@ -103,3 +103,9 @@ for (int i = 0; i < 100; ++i) {
     *cursor++ = ...; // Simple increment.
 }
 ```
+
+## Rule 3: Constants/Read-Only Data
+
+Access these values directly, like `context->max_buffer_size` or
+`context->exception_level`. If we never write to it, the compiler knows it
+will not change. It will load once and get cached automatically.