Add documentation for Splunk General Terms in both helm charts

[rlieberman-splunk]

Description

This pull request makes several documentation improvements to clarify how to set the SPLUNK_GENERAL_TERMS environment variable when deploying or upgrading the Splunk Operator, and updates code blocks for better formatting and usability.

Key Changes

• Added a prominent note in docs/Helm.md to clarify that SPLUNK_GENERAL_TERMS defaults to an empty string and must be manually set, with a reference to migration instructions.
• Expanded instructions in docs/SplunkGeneralTermsMigration.md to cover both splunk/splunk-operator and splunk/splunk-enterprise Helm charts for setting the SPLUNK_GENERAL_TERMS value.

Testing and Verification

• make docs-preview showed correct formatting

Related Issues

N/A

PR Checklist

• Code changes adhere to the project's coding standards.
• Relevant unit and integration tests are included.
• Documentation has been updated accordingly.
• All tests pass locally.
• The PR description follows the project's guidelines.