Extend monitoring and update portfolio

Add hostmetrics receiver and OTel Collector telemetry, and move
Alertmanager and Blackbox scraping to the collector. Raise collector
memory limits, restrict Blackbox to HTTP 200, and add collector and
Caddy down alerts with inhibition. Update portfolio title and copy.

docker-compose.observability.yml

@@ -57,10 +57,16 @@ services:
         configs:
             - source: otel_collector_config
               target: /etc/otelcol-contrib/config.yaml
+        volumes:
+            - /:/hostfs:ro
+            - /proc:/hostfs/proc:ro
+            - /sys:/hostfs/sys:ro
+            - /dev:/hostfs/dev:ro
+            - /run:/hostfs/run:ro
         deploy:
             resources:
                 limits:
-                    memory: 256M
+                    memory: 512M
         networks:
             - prometheus_network