Add write permissions to doxygen generation workflow on main (FreeRTOS#200)

Co-authored-by: czjaso <czjaso@amazon.com>

Author: jasonpcarroll

.github/workflows/doxygen.yml

@@ -1,4 +1,9 @@
 name: Doxygen Generation
+
+# Needs this to write to gh-pages branch.
+permissions:
+  contents: write
+
 on:
   push:
     branches: [main]