doc: gh_utils: add docstring for get_page_prefix

Add documentation to the get_page_prefix method.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

doc/_extensions/zephyr/gh_utils.py

@@ -51,6 +51,20 @@ __version__ = "0.1.0"
 
 
 def get_page_prefix(app: Sphinx, pagename: str) -> str:
+    """Return the prefix that needs to be added to the page path to get its location in the
+    repository.
+
+    If pagename refers to a page that is automatically generated by Sphinx or if it matches one of
+    the patterns in ``gh_link_exclude`` configuration option, return None.
+
+    Args:
+        app: Sphinx instance.
+        pagename: Page name (path).
+
+    Returns:
+        Prefix if applicable, None otherwise.
+    """
+
     if not os.path.isfile(app.env.project.doc2path(pagename)):
         return None