fix(cost-center): add missing sub-resources to website sidebar

The cost center sub-resources (organizations, repositories, users)
were registered in provider.go and had docs, but were accidentally
omitted from the website sidebar navigation.

Author: vmvarela

website/github.erb

@@ -316,6 +316,15 @@
             <li>
               <a href="/docs/providers/github/r/enterprise_cost_center.html">github_enterprise_cost_center</a>
             </li>
+            <li>
+              <a href="/docs/providers/github/r/enterprise_cost_center_organizations.html">github_enterprise_cost_center_organizations</a>
+            </li>
+            <li>
+              <a href="/docs/providers/github/r/enterprise_cost_center_repositories.html">github_enterprise_cost_center_repositories</a>
+            </li>
+            <li>
+              <a href="/docs/providers/github/r/enterprise_cost_center_users.html">github_enterprise_cost_center_users</a>
+            </li>
             <li>
               <a href="/docs/providers/github/r/enterprise_organization.html">github_enterprise_organization</a>
             </li>