Add more relevant links out to closed source features in our open source version

[dabeeeenster]

We sometimes have folk running Flagsmith open source becoming confused about what is open and what isn't - a lot people get sent the link to this page!

We should have a consistent UI experience to make sure that closed features are included in the open source version with relevant links out to the relevant docs area explaining what plan / version you need to be running to get access to that feature.