docs: fix rtd ad placement (canonical#1414)

Fixing the issue where the ad doesn't show up on the left navigation bar even if there is enough space for it.

docs/custom_conf.py

@@ -141,6 +141,14 @@
 # Addons-by-default, see: https://about.readthedocs.com/blog/2024/07/addons-by-default/
 if os.environ.get('READTHEDOCS', '') == 'True':
     html_context['READTHEDOCS'] = True
+    # The following are needed, see: https://github.com/pradyunsg/furo/blob/main/docs/conf.py#L135.
+    html_context['current_version'] = 'latest'
+    html_context['conf_py_path'] = '/docs/'
+    html_context['display_github'] = True
+    html_context['github_user'] = 'canonical'
+    html_context['github_repo'] = 'operator'
+    html_context['github_version'] = 'main'
+    html_context['slug'] = 'operator'
 
 # If your project is on documentation.ubuntu.com, specify the project
 # slug (for example, "lxd") here.