Creating a Teams Tab
It has always been pretty straightforward to do this. You just click on the little "+" icon at the end of the tabs:
If you have some web content you want to render, you can just use the Website built-in tab:
Or you could launch the Teams AppStudio and create a manifest that would give you a Teams application that you could distribute. Or you could go into full-on developer mode and use the Yeoman teams generator to build a full solution including a tab that surfaces your web content.