[DOC] Update the Longhorn document in Install with Helm

[roger-ryao]

Is your improvement request related to a feature? Please describe

The users follow the Longhorn document in Install with Helm.They could not helm install previous releases version of Longhorn.

Describe the solution you'd like

I want to update the Longhorn document in Install with Helm.
After that, users could follow the Longhorn document in Install with Helm and helm install a specific version of Longhorn.

Describe alternatives you've considered

NA

Additional context

helm install longhorn longhorn/charts/longhorn --namespace longhorn-system --create-namespace --version 1.2.4

[innobead]

cc @longhorn/qa this is good to have QA contribute to the doc as well.

[longhorn-io-github-bot]

Pre Ready-For-Testing Checklist

• Where is the reproduce steps/test steps documented?
  The reproduce steps/test steps are at:

• Is there a workaround for the issue? If so, where is it documented?
  The workaround is at:

• Does the PR include the explanation for the fix or the feature?

• Does the PR include deployment change (YAML/Chart)? If so, where are the PRs for both YAML file and Chart?
  The PR for the YAML change is at:
  The PR for the chart change is at:

• Have the backend code been merged (Manager, Engine, Instance Manager, BackupStore etc) (including backport-needed/*)?
  The PR is at

• Which areas/issues this PR might have potential impacts on?
  Area
  Issues

• If labeled: require/LEP Has the Longhorn Enhancement Proposal PR submitted?
  The LEP PR is at

• If labeled: area/ui Has the UI issue filed or ready to be merged (including backport-needed/*)?
  The UI issue/PR is at

• If labeled: require/doc Has the necessary document PR submitted or merged (including backport-needed/*)?
  The documentation issue/PR is at

• If labeled: require/automation-e2e Has the end-to-end test plan been merged? Have QAs agreed on the automation test case? If only test case skeleton w/o implementation, have you created an implementation issue (including backport-needed/*)
  The automation skeleton PR is at
  The automation test case PR is at
  The issue of automation test case implementation is at (please create by the template)

• If labeled: require/automation-engine Has the engine integration test been merged (including backport-needed/*)?
  The engine automation PR is at

• If labeled: require/manual-test-plan Has the manual test plan been documented?
  The updated manual test plan is at

• If the fix introduces the code for backward compatibility Has a separate issue been filed with the label release/obsolete-compatibility?
  The compatibility issue is filed at

[roger-ryao]

• 1.1.0
• 1.1.1
• 1.1.2
• 1.1.3
• 1.2.0
• 1.2.1
• 1.2.2
• 1.2.3
• 1.2.4
• 1.2.5
• 1.2.6
• 1.3.0
• 1.3.1
• 1.3.2
• 1.3.3
• 1.4.0
  Result Passed