docs: add DisaggregatedSet documentation (#811)

* docs: add DisaggregatedSet documentation

- Added concept, installation, reference, and examples

- Linked KEP-766

- Added navigation updates

* docs: address review feedback on DisaggregatedSet documentation

- Switch disaggregatedset.v1.md from hand-written to auto-generated
  via genref by adding api/disaggregatedset/v1/doc.go and updating
  hack/genref/config.yaml with the disaggregatedset API entry

- Document DisaggregatedSet labels, annotations in reference page:
  disaggregatedset.x-k8s.io/{name,role,revision,initial-replicas}

- Fold standalone disaggregatedset installation page into the main
  installation guide as an 'Enabling DisaggregatedSet' section;
  delete site/content/en/docs/installation/disaggregatedset.md

- Fix webhook verification command to use the correct webhook name:
  lws-validating-webhook-configuration

- Fix child LWS naming in examples to document the revision-hash
  format (<name>-<hash>-<role>) and use label selectors for queries

- Align overview DisaggregatedSet section with README 'Advanced
  DisaggregatedSet Features' bullet points

- Update all internal links pointing to the deleted installation page
  to /docs/installation/#enabling-disaggregatedset

* docs: address final review feedback from yankay

- concepts/disaggregatedset: remove manual TOC to align with existing
  concept pages (rollout-strategy, failure-handling) that rely on
  Docsy's right-side outline
- concepts/disaggregatedset: soften 'roles are independent' wording to
  reflect that DisaggregatedSet coordinates lifecycle and rollouts
  across roles as a cohesive unit
- overview: remove 'autoscaling' from DisaggregatedSet description;
  KEP-766 lists HPA/VPA autoscaling as out of scope and the CRD does
  not expose a scale subresource
- overview: remove the link list at the end of the DisaggregatedSet
  section; sidebar already covers all those pages
- installation: rename '## Enabling DisaggregatedSet' to
  '## DisaggregatedSet' and rewrite per yankay's suggested wording —
  clarifying that kubectl/Kustomize users need no extra steps, and that
  for Helm only the webhook and ClusterRoles are optional
- installation: bump all version examples from v0.8.0 to v0.9.0
- fix internal anchor links from #enabling-disaggregatedset to
  #disaggregatedset across concepts and examples pages