Add details about DNN DI Extensions like TryAdd

[valadas]

Description of documentation change

Add details about registering services that may or may not yet be registered (TryAdd) which is in DotNetNuke.DependencyInjection.Extensions

Proposed area for new documentation (if applicable)

It would be nice to tag along the area when #641 is merged.

Additional context

I can submit that, just waiting to get the section to exist to append to it.

[valadas]

Actually, this is in Microsoft.Extensions.DependencyInjection.Extensions

Would still be nice to show an example usage though as DNN does not have all of its interfaces registered.