doc: generate sitemap.xml

Generate a sitemap.xml for the documentation.
This aims at helping search engines react faster to the addition/removal
of new pages.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>
diff --git a/doc/requirements.txt b/doc/requirements.txt
index 9655cc6..983a29d 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -9,6 +9,7 @@
 sphinx-notfound-page
 sphinx-copybutton
 sphinx-togglebutton
+sphinx-sitemap
 
 # YAML validation. Used by zephyr_module.
 PyYAML>=6.0