Use monospace font for signatures (#134)

* chore: Set monospace for attr-name and attr-value

* Use monospace for method headings

ref. https://github.com/rails/sdoc/blob/e26cca7ccbd0a2cd9ee5a9570c20fa193c825d1a/lib/sdoc/helpers.rb#L209-L218

Author: toshimaru

src/_sass/railsdoc.scss

@@ -170,6 +170,14 @@ code {
   }
 }
 
+// FIXME: Wrap method with `code` tag in sdoc
+.method {
+  h3 {
+    font-family: monospace;
+    font-size: 1.3rem;
+  }
+}
+
 #content {
   $hacky-div-height: 4rem;
 
@@ -195,6 +203,15 @@ code {
 
   .attr-name {
     font-weight: bold;
+    font-family: monospace;
+  }
+
+  .attr-value {
+    font-family: monospace;
+  }
+
+  .attr-rw {
+    font-family: monospace;
   }
 }