Migrate demos with external plugins to new installation methods

[f1ames]

This PR migrates demos with 3rd-party plugins to NIM. Follow-up of #38:

I can prepare separate branch migrating all demos with external plugins which will be waiting to be merged. So we have still all demos fully functional (and fully featured) on master and then will be ready when those external plugins are migrated.

It's based on new-installation-methods-v2 branch (#38 PR) and I followed all the same conventions and assumptions. You can read #38 description for all details.

The main difference here is that 3rd-party plugins (WProofreader and Mathtype) are commented out in CKEditor 5 plugins list in it's config in each demo. This way demos can be run and after plugins are migrated to NIM, it should be enough to just uncomment those lines.