Extend documentation sources.

[Daniel Meza]

There are severals issues asociated to multiple documentation files or missed documentation files.

NSwag: 2161
Namotion.Reflection: 33

We need to provide a way to include documentation files sources or just add it to the Cache.

[Daniel Meza]

I have made a workarround here

[Rico Suter]

I think it would make sense to make the cache or a method public to register custom documents....