Clarify that examples are intended to run in dash

[alexcjohnson]

Some of the examples look like they could plausibly run as individual notebook cells (like plotly.py direct usage). We should clarify early on in the docs that all examples are partial or complete dash apps, and in specific cases where we only show the code for a component (like dash-bio, dash-table) we should reiterate this point at the beginning of the section. https://github.com/plotly/dash-bio/issues/446#issuecomment-562268337