(docs) Add example: kfrederix/single-spa-angular-esm

[kfrederix]

Hello,

First of all: thank you for creating and maintaining single-spa! We have been using it for the past 4 years in the company I work for, and I am a big fan!

I have now published an example Nx monorepo, demonstrating how single-spa can be used for microfrontends with the new Angular builder based on Vite/esbuild. It leverages native browser importmaps, ES modules and makes use of import-map-injector to add support for having multiple/external importmaps.

I have also written extensive documentation about specific issues/problems I faced while working on this example.

I would love to add this to the Examples section on the single-spa docs, if you think it could be helpful to others.

[MilanKovacic]

Thank you for the PR. Example looks valuable!