[REFACTOR] version-comparator: simplify the way pages are managed

[tbouffard]

Notes

• this only applies to pages that use the bpmn-visualization IIFE bundle from the npm package
• former version needs customization and the js files must be stored in the repository as it is not publicly available in a registry or package manager

Currently, adding a new version requires to duplicate a lot of content with an existing one.
In addition, we load the js bpmn-visualization file from a CDN: the file is not cached between executions (a new browser context is created each time the script start) and it prevents to work offline. This is the same problem as for #8

Proposal

As all pages are almost the same, we could create a template and use it to generate pages for all versions.
The js file could be retrieved once and put in a cache directory in the repository and reuse when available. We can start with a very basic implementation without checksum checks (but we can also do it and use existing cache lib).
Versions to consider could be stored in a declarative data structure, used to generate the content.