Fold onlyoffice chart into nextcloud-onlyoffice

We currently have a somewhat elaborate setup, with a top-level nextcloud-onlyoffice chart that has onlyoffice-documentserver as a requirement, which is specified to be fetched from an abstract @onlyoffice-documentserver helm repo, which needs to be created during installation to point to the onlyoffice-documentserver of this same git repo.

The idea of this construction is to enable installation of the onlyoffice chart by itself, without Nextcloud. This is commendable, but as we're not really advertising this possiblity, the benefits are thus far theoretical, while the complex dependency setup makes manual installation of the nextcloud-onlyoffice chart hard, and inclusion in the flux-based automatic update system seemingly impossible.

So let's make things easier for ourselves and end-users of this chart, and fold the two charts into one.