This address is a NuGet feed, for Visual Studio and dotnet rather than for a browser.
The source to add is:
https://nuget.cendia.com/index.json
Tools > Options > NuGet Package Manager > Package Sources, add a source named
cendia with the address above. Cendia packages and their updates then appear in
Manage NuGet Packages.
dotnet nuget add source https://nuget.cendia.com/index.json --name cendia
Documentation and release notes are at arena.cendia.com.