Add VTK-m User's Guide to source

This is a start of moving the VTK-m User's Guide into the VTK-m source. This is only the start of the process. There are several goals of this work.

1. Integrate the documentation into the source code better to better keep the code up to date.
2. Move the documentation over to Sphinx so that it can be posted online and be more easily linked.
3. Incoporate Doxygen into the guide to keep the documentation consistent.
4. Build the user guide examples as part of the VTK-m CI to catch compatibility changes quickly.