quarto: configure as "site" only with multiple render targets

[aronatkins]

Bundles should not be marked as "site" when there is only one render target.

https://github.com/rstudio/rsconnect-python/blob/18bfc5a5ac5b0ea03a2b9610dd04685e3919bd7f/rsconnect/bundle.py#L98-L103
https://github.com/rstudio/rsconnect-python/blob/18bfc5a5ac5b0ea03a2b9610dd04685e3919bd7f/rsconnect/bundle.py#L328-L334

Single-document projects are incorrectly marked as site. The existence project configuration is not enough to call the bundle a "site".