Allow the "More details about this document" to start collapsed

[jyasskin]

If you put the "More details about this document" inside a <details> instead of a <details open>, pubrules fails with

The <details> element in the header should have attribute "open".

Even though https://www.w3.org/pubrules/doc/rules?profile=NOTE#docIDFormat doesn't mention this requirement.

Similar to #1919, I'd like https://w3ctag.github.io/explainers/ to start with its 'More details' section collapsed, so that readers can focus on the document's actual content rather than its metadata.

[deniak]

I think this is reasonable. We already use local storage to save the state of the widget.
@plehegar WDYT?