Add a troubleshooting index page

This just lists the already existing docs under
doc/administration/troubleshooting

doc/administration/troubleshooting/index.md

+# Troubleshooting a GitLab installation
+
+Below are some resources to help you troubleshoot a GitLab installation
+in case something goes wrong:
+
+- [Debugging tips](debug.md)
+- [Diagnostics tools](diagnostics_tools.md)
+- [Elasticsearch](elasticsearch.md)
+- [GitLab Rails console cheat sheet](gitlab_rails_cheat_sheet.md)
+- [Group SAML and SCIM troubleshooting](group_saml_scim.md) **(SILVER ONLY)**
+- [Kubernetes cheat sheet](kubernetes_cheat_sheet.md)
+- [Linux cheat sheet](linux_cheat_sheet.md)
+- [Parsing GitLab logs with `jq`](log_parsing.md)
+- [Navigating GitLab via Rails console](navigating_gitlab_via_rails_console.md)
+- [PostgreSQL](postgresql.md)
+- [Sidekiq](sidekiq.md)
+- [SSL](ssl.md)
+
+If you need a testing environment to troubleshoot, see the
+[apps for a testing environment](test_environments.md).