Split concepts into subsections

Following the same basic idea used for guides and architecture, split
concepts section in subsections.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>

content/concepts/application.md

++++
+date = "2024-06-12"
+weight = 100
+
+title = "Application"
++++
+
+{{< section-toc >}}

content/concepts/application_customization.md

++++
+date = "2024-06-12"
+weight = 100
+
+title = "Application customization"
++++
+
+{{< section-toc >}}