docs(example): move config.yaml to config folder (#265)

ref: https://github.com/TwiN/gatus/issues/151#issuecomment-912932934

update all exmaple in docker-compose file.

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

.examples/docker-compose-mattermost/config/config.yaml

@@ -0,0 +1,16 @@
+alerting:
+  mattermost:
+    webhook-url: "http://mattermost:8065/hooks/tokengoeshere"
+    insecure: true
+
+endpoints:
+  - name: example
+    url: https://example.org
+    interval: 1m
+    alerts:
+      - type: mattermost
+        enabled: true
+        description: "health check failed 3 times in a row"
+        send-on-resolved: true
+    conditions:
+      - "[STATUS] == 200"