docs: Fix centering of images.

Signed-off-by: Tim 'mithro' Ansell <tansell@google.com>

docs/_static/extra.css

diff --git a/docs/_static/extra.css b/docs/_static/extra.css
index f43d2fc..c9b2423 100644
--- a/docs/_static/extra.css
+++ b/docs/_static/extra.css
@@ -1,3 +1,7 @@
+img.align-center {
+	display: block;
+}
+
 span.lib-process {
 	color: green;
 }