docs: document safe Helm upgrade path for CRDs (#883)

* docs: document safe Helm upgrade path for CRDs

Helm installs files under the chart's crds/ directory only on the
initial install and never updates or deletes them on helm upgrade, so
CRD schema changes and newly added CRDs do not reach existing releases
through helm upgrade alone.

Document the supported in-place upgrade procedure in both the chart
README and the site installation docs: apply the CRDs with
kubectl apply --server-side --force-conflicts before running
helm upgrade, covering both the leaderworkerset and disaggregatedset
CRDs. Also warn against the uninstall-then-install workaround, which
deletes the CRDs and cascades to the deletion of all custom resources.

* docs: correct inaccurate uninstall note for crds/-directory CRDs

Helm never updates or deletes CRDs placed in the crds/ directory —
they are outside the release manifest, so helm upgrade and helm uninstall
leave them untouched. The previous Note incorrectly claimed that
`helm uninstall` would delete the CRDs. Update both the Helm chart README
and the installation guide to reflect actual Helm behavior.

* docs: document one-time keep annotation for pre-v0.8.0 Helm upgrades