Update css to visually differentiate h4, h5, h6 in markdown.

They're currently styled the same which makes:
  https://fuchsia.googlesource.com/tracing/+/master/docs/trace_format.md
confusing to read.

My proposed styling changes are subtle but there's not a lot of room to
work with.

Change-Id: I7d4583d6d61e621b24ae3cf0a8f4ec9a4f5d69d6

gitiles-servlet/src/main/resources/com/google/gitiles/static/doc.css

diff --git a/gitiles-servlet/src/main/resources/com/google/gitiles/static/doc.css b/gitiles-servlet/src/main/resources/com/google/gitiles/static/doc.css
index 3006d29..a531087 100644
--- a/gitiles-servlet/src/main/resources/com/google/gitiles/static/doc.css
+++ b/gitiles-servlet/src/main/resources/com/google/gitiles/static/doc.css
@@ -43,8 +43,16 @@
 .doc h3 {
   font-size: 1.3em;
 }
-.doc h4,
-.doc h5,
+.doc h4 {
+  font-size: 1.3em;
+  font-weight: lighter;
+  font-style: italic;
+}
+.doc h5 {
+  font-size: 16px;
+  font-weight: lighter;
+  font-style: italic;
+}
 .doc h6 {
   font-size: 14px;
   font-style: italic;