Refactoring of docs

[smaillet]

• Moved docs markdown for samples into the actual sample libraries to make it easier to find and provide contextual docs when dealing only in source.
• Editorial corrections and updates to doc markdown topics
• Kaleidoscope Changed Chapter 2 to conform with Program+ReplEngine+CodeGenerator pattern of the other chapters to make discussion and comparisons easier.

Breaking Change

• OrcJit lazy call back support is currently disabled due to bugs in LLVM impacting the JIT on Windows
• To help prevent accidental misuse of the LLVM target registration (e.g. register before initialization) the library initialization and registration were changed. The registration static methods were all removed and instead an interface for registration is provided. The initialize now returns an instance of that interface, which is also IDisposable. The interface is then used to register the targets. This ensures that the registration only occurs after initialization. Additionally, the logic for initialization was updated to defend against any sort of init.dispose,re-init that isn't supported by the underlying LLVM library. (Both of these points were the results of attempts to document the correct use and reallizing that a better answer was to enforce the pattern in the library itself rather than relying on the docs.)