feat(docs): add complete Vue 2 and Vue 3 examples to docs

### open questions:
- Should/can the example be interactive and live in an example section?
- Should the example be a different kind of mini file browser where you see file name list at the right and the file content on the left?
- Should the example include user auth (because in eg. a todo list this would be a requirement)
- Should I link to repo links to Vue 2 / Vue 3 / Vue3+Firestore examples?
- Should I add example repo with Firestore+Vue2?