Add details about DNN DI Extensions like TryAdd

## 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.