Update autotuning documentation to reflect current interface and gh-based workflow (#688)

- Updated to reflect current AutotuneResults object structure
- Documented the new simplified interface (autotune_setup returns AutotuneResults)
- Added comprehensive examples for the plot() and share_results() functions
- Clarified the current size categories (:tiny, :small, :medium, :large, :big)
- Updated GitHub authentication instructions for both gh CLI and token methods
- Added detailed examples for customizing benchmarks
- Included troubleshooting section for common issues
- Documented the community sharing workflow via GitHub issue #669
- Clarified that preference setting is still under development
- Added examples for advanced usage patterns

Co-authored-by: ChrisRackauckas <[email protected]>

Author: ChrisRackauckas-Claude