Properly render ACL in documentation

[lkiesow]

The ACL example in the documentation specifies the highlighting as JSON, but it has comments which causes GitHub to render them as errors. Comments are allowed in the JSON5 extension, however, to which this patch switches.

Here is how this looks before and after the change:

• have read the CONTRIBUTING.md file
• raised a GitHub issue or discussed it on the projects chat beforehand
• added unit tests
• added integration tests
• updated documentation if needed
• updated CHANGELOG.md

[nblock]

Unfortunately, this breaks the rendering in mkdocs:

With json5 (broken)

With json (ok)