Provide documentation for the ArgoCD dynamic plugin.

For reference, this was previously documented in RHPIB (precursor to RHDH) but this content will need to be reviewed and updated particularly with regard to the installation procedure.

https://access.redhat.com/documentation/en-us/red_hat_plug-ins_for_backstage/2.0/html-single/argocd_plugin_for_backstage/index#argocd-plugin-for-backstage