Allow for a gitlab social media button (#289)

Alongside the existing github option.

Author: pgjones

docs/user_guide/configuring.rst

@@ -29,6 +29,7 @@ following configuration:
 
    html_theme_options = {
      "github_url": "https://github.com/<your-org>/<your-repo>",
+     "gitlab_url": "https://gitlab.com/<your-org>/<your-repo>",
      "twitter_url": "https://twitter.com/<your-handle>",
    }
 

pydata_sphinx_theme/docs-navbar.html

@@ -40,6 +40,13 @@
             </a>
           </li>
         {% endif %}
+        {% if theme_gitlab_url | length > 2 %}
+          <li class="nav-item">
+            <a class="nav-link" href="{{ theme_gitlab_url }}" target="_blank" rel="noopener">
+              <span><i class="fab fa-gitlab-square"></i></span>
+            </a>
+          </li>
+        {% endif %}
         {% if theme_twitter_url | length > 2 %}
           <li class="nav-item">
             <a class="nav-link" href="{{ theme_twitter_url }}" target="_blank" rel="noopener">