Support later nsdoc revision

[danyill]

Is your feature request related to a problem? Please describe.

As a user of OpenSCD I want to use the latest nsd/nsdocs file.

At present it seems that we are looking for a match to the files in packages/openscd/public/xml which (at least one) is out of date.

Users are trying to use later versions and finding that they cannot be used. https://openscd.zulipchat.com/#narrow/channel/447335-general/topic/NSDocs/near/484782598

Describe the solution you'd like

OpenSCD needs a more flexible approach and the latest nsd files should be included.

Requirements

Ideally, support arbitrary version, revision and release in the OpenSCD code and then periodically we update the nsd files.