docs: concepts and building_applications migration (#3534)

# What does this PR do?

- Migrates the remaining documentation sections to the new documentation format

<!-- Provide a short summary of what this PR does and why. Link to relevant issues if applicable. -->

<!-- If resolving an issue, uncomment and update the line below -->

<!-- Closes #[issue-number] -->

## Test Plan

- Partial migration

<!-- Describe the tests you ran to verify your changes with result summaries. *Provide clear instructions so the plan can be easily re-executed.* -->

docs/source/distributions/k8s/postgres-k8s.yaml.template

@@ -1,66 +0,0 @@
-apiVersion: v1
-kind: PersistentVolumeClaim
-metadata:
-  name: postgres-pvc
-spec:
-  accessModes:
-    - ReadWriteOnce
-  resources:
-    requests:
-      storage: 10Gi
----
-apiVersion: apps/v1
-kind: Deployment
-metadata:
-  name: postgres
-spec:
-  replicas: 1
-  selector:
-    matchLabels:
-      app.kubernetes.io/name: postgres
-  template:
-    metadata:
-      labels:
-        app.kubernetes.io/name: postgres
-    spec:
-      containers:
-      - name: postgres
-        image: postgres:15
-        env:
-        - name: POSTGRES_DB
-          value: "${POSTGRES_DB}"
-        - name: POSTGRES_USER
-          value: "${POSTGRES_USER}"
-        - name: POSTGRES_PASSWORD
-          value: "${POSTGRES_PASSWORD}"
-        - name: PGDATA
-          value: "/var/lib/postgresql/data/pgdata"
-        ports:
-        - containerPort: 5432
-        resources:
-          requests:
-            memory: "512Mi"
-            cpu: "250m"
-          limits:
-            memory: "1Gi"
-            cpu: "500m"
-        volumeMounts:
-        - name: postgres-storage
-          mountPath: /var/lib/postgresql/data
-      volumes:
-      - name: postgres-storage
-        persistentVolumeClaim:
-          claimName: postgres-pvc
----
-apiVersion: v1
-kind: Service
-metadata:
-  name: postgres-server
-spec:
-  selector:
-    app.kubernetes.io/name: postgres
-  ports:
-  - protocol: TCP
-    port: 5432
-    targetPort: 5432
-  type: ClusterIP