Mention to update the Nginx configuration when upgrading to OpenCloud 8.x.x.

[eriktews]

The Nginx configuration template of OpenCloud 7.x.x doesn't have a dedicated location block enabling web sockets for the yjs collaboration service yet and without such a block, web socket connections can't be established and collaborative editing won't work with OpenCloud 8.x.x behind an external Nginx reverse proxy. This commit adds a paragraph about this to the upgrade guide.

[Svanvith]

Thanks a lot for creating this PR and for pointing out the required Nginx changes for Yjs.

I created #1181 to document the Nginx configuration in the external proxy documentation instead.

The main reason is to avoid maintaining the same proxy configuration in multiple places. The upgrade guide should only contain a short note for administrators who have explicitly enabled Yjs, since Yjs is optional and only required for real-time collaborative editing in the OpenCloud Editor.

The full /yjs WebSocket configuration is better kept in the external proxy documentation as the single source of truth, while the upgrade guide can link to it when Yjs is in use.

I'll therefore close this PR in favor of #1181. Thanks again for the contribution and for bringing this up.