doc: extensions: zephyr-domain: make Breathe inserts optional

So that external users of the domain only interested in e.g. referencing
roles, can skip tweaks made to Breathe's directives.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>
diff --git a/doc/conf.py b/doc/conf.py
index f8bfb06..808df88 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -310,6 +310,10 @@
     "build/dts/api/compatibles/**/*",
 ]
 
+# -- Options for zephyr.domain --------------------------------------------
+
+zephyr_breathe_insert_related_samples = True
+
 # -- Options for sphinx.ext.graphviz --------------------------------------
 
 graphviz_dot = os.environ.get("DOT_EXECUTABLE", "dot")