doc: mention usage of .readthedocs.yml

Mention that HTML pages are published at https://u-boot.readthedocs.io/
and that the file .readthedocs.yml controls the build process.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/build/documentation.rst b/doc/build/documentation.rst
index 896264d..011cd34 100644
--- a/doc/build/documentation.rst
+++ b/doc/build/documentation.rst
@@ -26,6 +26,9 @@
     # Display the documentation in a graphical web browser
     x-www-browser doc/output/index.html
 
+The HTML documentation is published at https://u-boot.readthedocs.io. The build
+process for that site is controlled by the file *.readthedocs.yml*.
+
 Infodoc documentation
 ---------------------