Only let browser search through source code until it's expanded

Something that's been bothering me is that while the source code is
not visible by default, the browser still jump to it when searching.
Adding the `visible` property prevents this.

Test it out yourself:
* `bundle exec rdoc`
* open `_site/index.html`
* Search for `NameError`

Before, you will get a match from `load_yaml` source code, after
you only get the match when that methods source code is expanded.

lib/rdoc/generator/template/darkfish/css/rdoc.css

@@ -585,6 +585,9 @@ main header h3 {
 /* @group Method Details */
 
 main .method-source-code {
+  /* While this is already invisible through the rule below, this will inform the browser to
+  not consider source code during text searching until it is actually expanded. */
+  visibility: hidden;
   max-height: 0;
   overflow: auto;
   transition-duration: 200ms;
@@ -594,6 +597,7 @@ main .method-source-code {
 }
 
 main .method-source-code.active-menu {
+  visibility: visible;
   max-height: 100vh;
 }