docs: Add example of docker-compose with multiple configuration files

.examples/docker-compose-multiple-config-files/config/backend.yaml

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+endpoints:
+  - name: check-if-api-is-healthy
+    group: backend
+    url: "https://twin.sh/health"
+    interval: 5m
+    conditions:
+      - "[STATUS] == 200"
+      - "[BODY].status == UP"
+      - "[RESPONSE_TIME] < 1000"
+
+  - name: check-if-website-is-pingable
+    url: "icmp://example.org"
+    interval: 1m
+    conditions:
+      - "[CONNECTED] == true"
+
+  - name: check-domain-expiration
+    url: "https://example.org"
+    interval: 6h
+    conditions:
+      - "[DOMAIN_EXPIRATION] > 720h"

.examples/docker-compose-multiple-config-files/config/frontend.yaml

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+endpoints:
+  - name: make-sure-html-rendering-works
+    group: frontend
+    url: "https://example.org"
+    interval: 5m
+    conditions:
+      - "[STATUS] == 200"
+      - "[BODY] == pat(*<h1>Example Domain</h1>*)" # Check for header in HTML page

.examples/docker-compose-multiple-config-files/config/global.yaml

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+metrics: true
+debug: false
+ui:
+  header: Example Company
+  link: https://example.org
+  buttons:
+    - name: "Home"
+      link: "https://example.org"