docs/customizations: Move customization examples to dedicated folder

With the objective of improving our README, customization examples are being moved to a dedicated folder under `docs/`.

Signed-off-by: ArthurSens <arthursens2005@gmail.com>

docs/customizations/alertmanager-configuration.md

@@ -0,0 +1,40 @@
+### Alertmanager configuration
+
+The Alertmanager configuration is located in the `values.alertmanager.config` configuration field. In order to set a custom Alertmanager configuration simply set this field.
+
+```jsonnet mdox-exec="cat examples/alertmanager-config.jsonnet"
+((import 'kube-prometheus/main.libsonnet') + {
+   values+:: {
+     alertmanager+: {
+       config: |||
+         global:
+           resolve_timeout: 10m
+         route:
+           group_by: ['job']
+           group_wait: 30s
+           group_interval: 5m
+           repeat_interval: 12h
+           receiver: 'null'
+           routes:
+           - match:
+               alertname: Watchdog
+             receiver: 'null'
+         receivers:
+         - name: 'null'
+       |||,
+     },
+   },
+ }).alertmanager.secret
+```
+
+In the above example the configuration has been inlined, but can just as well be an external file imported in jsonnet via the `importstr` function.
+
+```jsonnet mdox-exec="cat examples/alertmanager-config-external.jsonnet"
+((import 'kube-prometheus/main.libsonnet') + {
+   values+:: {
+     alertmanager+: {
+       config: importstr 'alertmanager-config.yaml',
+     },
+   },
+ }).alertmanager.secret
+```