docs: render class="literal" with monospace font

When using ``....`` in RST, this results in <span class="literal">...</span>
instead of <code>...</code>. We thus need an extra rule to render it
with a monospace font. Colouring a light gray also helps the text
stand out a little more and matches background of <pre> blocks.
Reviewed-by: NJán Tomko <jtomko@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

docs/libvirt.css

@@ -574,3 +574,8 @@ ul.news-section-content li dl dd {
     margin-top: 0.5em;
     margin-bottom: 0.5em;
 }
+
+.literal, code {
+    font-family: monospace;
+    background: #eeeeee;
+}