css: Add scrollbar-gutter property to prevent shifted content

Some pages require a scroll bar, some do not. If the scroll bar is
hidden on a page that do not require it (default behavior of most
browsers), centered content is shifted to the right slightly.

Fix this behavior by using the scrollbar-gutter property, so the
renderer automatically reserves space for the scrollbar.

assets/_custom.scss

@@ -1,3 +1,7 @@
+html {
+	scrollbar-gutter: stable;
+}
+
 .book-page {
 	box-shadow: 0px -1.5rem 1.5rem rgba(0, 0, 0, 0.15);
 	padding-left: 2rem;