Documentation improvements

[maxkratt]

Specify on this page that network object prefabs nested under another network object prefab cannot be unparented.

Add support for a PDF download of the API Documentation as supported here: https://dotnet.github.io/docfx/docs/pdf.html

List prominent up to date tutorials somewhere on the docs such as this one: https://www.youtube.com/watch?v=F5VFEy1_THw

Update the broken Master Server Toolkit link here: https://fish-networking.gitbook.io/docs/manual/guides/add-ons
to their asset store page: https://assetstore.unity.com/packages/tools/network/master-server-toolkit-194832

Remove the Network Level of Detail section here: https://fish-networking.gitbook.io/docs/master/pro-learning-resources-and-support

Add the CanoeWebRTC transport to the transports page

Mention here that predicted spawning cannot spawn an object nested to another.

RollbackManager.PhysicsType.ThreeDimensional should be changed to RollbackPhysicsType.Physics on this page: https://fish-networking.gitbook.io/docs/manual/guides/lag-compensation/raycast

The first code block here should show the class inheriting from NetworkBehaviour to avoid confusing people.