Document generated widget options

[Kkkakania]

Summary

• document the widgets, widget_layout, and widget_type plotting options
• add executable examples for custom controls and scrubber mode
• clarify that widget options return a Panel layout
• correct the documented widget_location default to right_top

Closes #1108.

Validation

• .venv/bin/python -m pytest hvplot/tests/testpanel.py -q (3 passed)
• .venv/bin/python -m pytest --nbval-lax doc/ref/plotting_options/size_layout.ipynb -q (14 passed)
• git diff origin/main...HEAD --check