Frontend should be able serve nsdoc files

[pascalwilbrink]

Is your feature request related to a problem? Please describe.
Currently, the scl-validator is serving the nsdoc files. If you run CoMPAS without the scl-validator, you'll get errors saying that the nsdoc files cannot be loaded.
These NSDoc files are not strictly bound to the scl-validator as is to the frontend.
Therefore CoMPAS-openscd should serve the NSDoc files, instead of the scl-validator.

Requirements
The Dockerfile should have a volume for NSDoc files that can be exposed.