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.

[maximlt]

We decided not to document these options in details as there is a chance they will be deprecated/modified. Enabling them returns a Panel object instead of a HoloViews object, which we want to fix, and in doing so the API may change. So we don't want users starting to leverage them.

[Kkkakania]

Understood, thanks for the context. I agree that the reference should not encourage use of an API that may be deprecated or substantially changed, so I'm closing this PR.