add an edit button on Documentation pages

I successfully tested the change by playing with Chrome DevTools on bazel.build, as I could not find instructions about how to run the site locally :(.

Closes #2992.

PiperOrigin-RevId: 156349018

site/_layouts/documentation.html

diff --git a/site/_layouts/documentation.html b/site/_layouts/documentation.html
index c671b29..f15444c 100644
--- a/site/_layouts/documentation.html
+++ b/site/_layouts/documentation.html
@@ -107,6 +107,27 @@
           </nav>
         </div>
         <div class="col-lg-9">
+          <a id="gh-edit" class="default-hidden"><i class="fa fa-pencil" aria-hidden="true"></i> Edit</a>
+          <script>
+            var ghDocsBazeURL = 'https://github.com/bazelbuild/bazel/tree/master/site/versions/master/docs/';
+            var editButton = document.getElementById('gh-edit');
+            // if there is an edit button and we are on a doc page but not in the Build Encyclopedia
+            if(editButton
+              && window.location.pathname.split('docs/').length > 1
+              && window.location.pathname.lastIndexOf('/be/') == -1) {
+              var docFile = window.location.pathname.split('docs/')[1];
+              // some pages are now using markdown :(
+              if( docFile !== 'bazel-user-manual.html'
+              && docFile !== 'build-ref.html'
+              && docFile !== 'query.html'
+              && docFile !== 'test-encyclopedia.html') {
+                docFile = docFile.replace('html', 'md');
+              }
+              editButton.href = ghDocsBazeURL + docFile;
+              editButton.style.visibility = 'visible';
+            }
+          </script>
+
           {{ content }}
         </div>
       </div>