sphinx: theme_override: Use bold for emphasis text

This more closely matches the original docbook style and is appropriate
given the way the manual uses this element.

(From yocto-docs rev: d49ea9831dc7deccde34c63158afc114d8721857)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/sphinx-static/theme_overrides.css

@@ -74,6 +74,10 @@ h6 {
   font-weight: bold;
 }
 
+em {
+  font-weight: bold;
+}
+
 .pre {
   font-size: medium;
   font-family: Courier, monospace;