I just did a clean install of Visual Studio 2017 and the "Forms Content Page Xaml" showed up in Add New Item dialog. Which items you chose when installing the Xamarin for Visual Studio pack? Compare with the ones I've selected. Maybe something was left unchecked?
Also, if you want to keep the Xaml file together with the code-behind file, there's a way to do it by editing the YourProjectName.projitems inside your project folder. To find it, right-click the your main project and open its folder in the Windows Explorer, and then use a file editor to edit the .projitems file.
Search for the Compile tag for the .cs files you want and add the DependentUpon tag inside it, just like in the example below:
<Compile Include="$(MSBuildThisFileDirectory)YourFile.xaml.cs">
<DependentUpon>YourFile.xaml</DependentUpon>
</Compile>
After doing this, unload and reload your project.
Unfortunatelly I've had to do this procedure to all the new Xaml files I've created on my projects.