JSAdm: Docs for adding products to categories

First I want to thank you guys for the great project.

Couldn't find a documentation for the purpose and as a newbie here it took me quite some time to find the correct way of doing this:

```
curl -b cookies.txt -c cookies.txt \
-X PATCH 'http://localhost:8000/default/jsonadm/product/11?_token=...' \
-H 'Content-Type: application/json' \
-d '{"data": {
"type": "product",
"relationships": {
"catalog": {
"data": [{
"id": "48",
"type": "catalog"
}]
}
}
}}'
```

It adds product 11 to category 48. Not sure but I think it can go under [manage resources](https://aimeos.org/docs/2023.x/admin/jsonadm/manage-resources/)