Add a "demo mode" to the docs pages

[Akeboshiwind]

Add ?demo=true to the url and it will be activated

• Removes header, both sidebars & footer
• Makes anything other than the fiddle transparent

https://github.com/xtdb/xtdb/assets/8889986/7e6f7fb3-b0a5-49dc-a446-fe8d1f6c3d98

Works best on non-asciidoc pages

[aws-amplify-eu-west-1[bot]]

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