[Docs][C++] Guide to to work with RLE data (in kernels?)

Topics I can think of right now:

• introducing some utility functions for working with RLE data that I'm collecting currently as rle_util.h on my filter branch
• how physical/logical indices work, especially in combination with arrow offsets
• how common kinds of kernels can be converted to RLE. E.g scalar kernels with a single input could automatically be run on the internal data array of RLE. Scalar with multiple inputs and some kinds of vector algorithms should use VisitMergedRuns...