docs(helm): document the probes block, and survive probes being nulled

A probe map is exactly the value someone reaches for when their cluster is
unhappy, so the two ways of reaching for it should not break the render.
`probes: null` hit a nil pointer on `.Values.probes.liveness`; guarding with
`with (default dict ...)` also makes dropping a single probe work, which is what
you want on a pod whose liveness check is the thing killing it.

The rest is documentation the chart already gives every other value: a wiki
section next to Resource Limits, a Notes bullet, and a values comment naming the
one trap here, that a handler switch has to clear the shipped httpGet in the same
override or the API refuses the pod.

Verified with helm template: defaults render byte for byte what the chart
produced before, probes=null renders no probes, probes.liveness=null leaves one,
and helm lint passes.

charts/trek/templates/deployment.yaml

@@ -80,10 +80,14 @@ spec:
               mountPath: /app/data
             - name: uploads
               mountPath: /app/uploads
+          {{- with (default dict .Values.probes).liveness }}
           livenessProbe:
-            {{- toYaml .Values.probes.liveness | nindent 12 }}
+            {{- toYaml . | nindent 12 }}
+          {{- end }}
+          {{- with (default dict .Values.probes).readiness }}
           readinessProbe:
-            {{- toYaml .Values.probes.readiness | nindent 12 }}
+            {{- toYaml . | nindent 12 }}
+          {{- end }}
       volumes:
         - name: data
           {{- if .Values.persistence.enabled }}