Various CLV Quickstart notebook issues

[cluhmann]

• The link to the shifted BG model at the very top of the notebook is broken (points here
• There is no link to the BB/BG model (e.g., to here)
• The gamma gamma model is fit via gg.fit(fit_method="map"). I'm not sure why and it's not commented on in the notebook when used. We should not encourage such usage unless there is some good reason (I also see some usage elsewhere, e.g., here)