Fix #2835: Trigger rebuild when theme files change

[kishorhange111]

This PR addresses an issue where changes to theme files (templates and static assets) were not detected by Sphinx, requiring a manual make clean before rebuilding.

The change introduces a theme_hash in BuildInfo to track theme file changes. When the hash differs from the previous build, Sphinx detects the mismatch and triggers a rebuild automatically.

• Theme template changes now trigger a full rebuild.
• Static asset changes are detected without requiring manual cleanup.
• Existing behavior is preserved when no changes are detected.

Tests have been added to verify that theme changes correctly trigger rebuilds.

I have also tested this locally by modifying theme files and rebuilding without running make clean. Sphinx correctly detected the changes and triggered a rebuild, confirming the expected behavior.

Please let me know if any further adjustments would be helpful.

References

• Fixes Theme developers should be able to configure sphinx to trigger rebuilds based on theme changes #2835