[site] Center block diagram on documentation page

Signed-off-by: James Wainwright <james.wainwright@lowrisc.org>

site/landing/assets/scss/_block-diagram.scss

diff --git a/site/landing/assets/scss/_block-diagram.scss b/site/landing/assets/scss/_block-diagram.scss
new file mode 100644
index 0000000..8cea4f7
--- /dev/null
+++ b/site/landing/assets/scss/_block-diagram.scss
@@ -0,0 +1,3 @@
+.diagram {
+    margin: 2em auto;
+}

site/landing/assets/scss/single.scss

diff --git a/site/landing/assets/scss/single.scss b/site/landing/assets/scss/single.scss
index 49142a4..55cbda4 100644
--- a/site/landing/assets/scss/single.scss
+++ b/site/landing/assets/scss/single.scss
@@ -3,6 +3,7 @@
 @import "base";
 @import "content-body";
 @import "boxes";
+@import "block-diagram";
 @import "main-header";
 @import "scheme-toggle";
 @import "footer";