Document generated widget options

[Kkkakania]

Summary

• document the actual right_top default for generated widget placement
• add widgets, widget_layout, and widget_type to the public converter option reference
• add executable examples for a custom widget type, horizontal widget layout, and a multi-dimension scrubber

The new reference stays in the existing size and layout options page and does not overlap with the broader widget-guide reorganization in #1654.

Closes #1108.

Validation

• python -m pytest hvplot/tests/testpanel.py -q (3 passed)
• python -m pytest --nbval-lax -p no:python doc/ref/plotting_options/size_layout.ipynb -q (14 passed)
• pre-commit run --files hvplot/converter.py doc/ref/plotting_options/size_layout.ipynb (all hooks passed)
• jq empty doc/ref/plotting_options/size_layout.ipynb
• git diff --check

AI disclosure: OpenAI Codex assisted with repository inspection, the documentation changes, and local validation under Kkkakania's direction.