fix: build artefacts location

This merge request resolves #36 (closed) by removing the post-build script, which changed the location of the build output during deployment and caused the published specification website to return a 404 error. The output has been restored to its original location.

Note that when running npm run dev locally, the site is served from http://localhost:3000/unece/uncefact/spec-unvtd, not from http://localhost:3000. This is due to the Next configuration.

Depending on the preferences of the maintainers, it might be more developer-friendly to manage this configuration using an environment variable, so that when running the website locally, it is served from http://localhost:3000, but when deployed to production, it is served from https://un.opensource.unicc.org/unece/uncefact/spec-unvtd.