Expose startTimeout in Helm chart

[bluicezhen]

Add startTimeout to FE/BE/CN/broker specs so Helm values flow through to the DorisCluster CRD. Document the new options in values.yaml and README and note the default 300s behavior when unset.

What problem does this PR solve?

Issue Number: N/A

Related PR: N/A

Problem Summary:

• The Helm chart did not expose startTimeout, so DorisCluster’s CRD-supported setting could not be configured via Helm.
• This PR adds startTimeout to FE/BE/CN/broker values and templates, and documents the option and the default 300s behavior in values.yaml and the Helm README.

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label