`Assembly.Load` documentation was not updated for .NET Core/.NET

There are multiple issues I noticed from a quick read:
* Mentions of overloads which don't exist in .NET Core (For example the `Evidence` mentions)
* Samples using .NET 4 assembly versions
* `CodeBase` rules - which are probably different in .NET Core (in addition to the fact the
* References to https://docs.microsoft.com/en-us/dotnet/framework/deployment/how-the-runtime-locates-assemblies which is only about .NET Framework
* The docs should probably mention some key differences to .NET Framework - for example that probing on disk works differently (actually doesn't happen by default). https://github.com/dotnet/runtime/issues/62522