refactor(demo): reorganize CSS

[tbouffard]

Remove duplication. Move shared CSS files to specific subdirectories. Provide a single CSS entry point for each demo page. Update path-resolver.html to use the same layout for the bpmn-container.

[github-actions[bot]]

🎊 PR Preview 14926b4398e19a49a9706fe8bb210c4a1ca5990c has been successfully built and deployed to https://process-analytics-bv-experimental-add-ons-demo-pr-284.surge.sh

🕐 Build time: 0.011s

_{🤖 By surge-preview}

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud