feat: Add per-instance HA volumes

[as51340]

Summary

• add data-instance-specific extra volumes and mounts
• add coordinator-instance-specific extra volumes and mounts
• retain shared data and coordinator volume settings

Testing

• helm template test-release charts/memgraph-high-availability
• pre-commit run --files charts/memgraph-high-availability/templates/data.yaml charts/memgraph-high-availability/templates/coordinators.yaml charts/memgraph-high-availability/values.yaml

[as51340]

Tracking

• [Link to Epic/Issue]

Standard development

• Update / add chart templates, values, and NOTES.txt as needed
• Run helm lint and helm template locally against the changed chart(s)
• Verify the chart installs and upgrades cleanly on a test cluster

Labels checklist

• Add a docs label (exactly one): docs-changelog-only, docs-needed, or docs-not-needed
• Add at least one component label: memgraph, memgraph-ha, memgraph-lab, or infrastructure
• Add at least one type label: bug, feature, or infrastructure
• Assign the PR to a milestone
  • If not known, set for a later milestone

Documentation checklist

• Write a release note, including added/changed clauses
  • A chart users can mount extra volumes, including existing PVCs, on individual data instances or coordinators.[feat: Add per-instance HA volumes #281]
  • What has changed? What does it mean for a user? What should a user do with it? [#{{PR_number}}]({{link to the PR}})
• Documentation PR link
  • Is back linked to this development PR

[github-actions]

Release note advisory

A chart users can mount extra volumes, including existing PVCs, on individual data instances or coordinators.[#281]

The note has format and minor grammar issues: the PR link is just [#281] and must include the full URL as #281; the sentence reads "A chart users..." (should be e.g. "Chart users can mount..."), and there should be a space before the PR link; also remove the bold markup.