Docs: Populate empty index pages (#7200)

Three docs index pages are dead ends - they are either blank or have no
links. In lieu of more detailed content, this PR populates these pages
with `{{< section >}}` shortcodes that automatically generate TOC-like
links to subpages.

docs/sources/fundamentals/_index.md

@@ -4,3 +4,7 @@ weight: 150
 ---
 # Grafana Loki Fundamentals
 
+This section explains fundamental concepts about Grafana Loki:
+
+{{< section >}}
+

docs/sources/maintaining/_index.md

@@ -5,3 +5,6 @@ weight: 1200
 # Grafana Loki Maintainers' Guide
 
 This section details information for maintainers of Grafana Loki.
+
+{{< section >}}
+

docs/sources/operations/_index.md

@@ -4,3 +4,7 @@ weight: 800
 ---
 
 # Operations
+
+This section covers operational topics in Loki:
+
+{{< section >}}