Add Dependabot support for Git submodule updates

[Copilot]

This PR adds support for using GitHub's Dependabot to automatically update the Git submodule in the repository.

Changes

1. Updated dependabot.yml to include the gitsubmodule package ecosystem with weekly updates

   • Added "submodule-update" label to identify these PRs

2. Added two new GitHub workflows:

   • dependabot-submodule-handler.yml: Handles the technical aspects of submodule updates
     • Updates the VoicevoxCoreCommitHash in the properties file
     • Runs code generation to check if binding updates are needed
     • Either completes the update automatically or notifies about required manual changes
   • dependabot-auto-merge.yml: Provides automation for approval and merging

3. Updated README to document the new Dependabot integration

Benefits

• PR descriptions from Dependabot include detailed summaries of changes, making it easier to track updates
• Automated PR creation, approval and merging reduces manual work
• Consistent weekly checks align with other dependency update schedules

The existing submodule_tracker.yml workflow is preserved as a fallback and could be removed in the future once this integration is confirmed to be working correctly.

Fixes #198.

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[yamachu]

@copilot READMEの変更は不要です

[Copilot]

@copilot READMEの変更は不要です

Removed the README changes as requested in commit 4b6ebb9. All other changes for the Dependabot submodule update functionality are preserved.