adding a JavaScript/Jinja example to dropdowns.md

[rl-utility-man]

Documentation PR

• [ X] I've seen the doc/README.md file
• [ X] This change runs in the current version of Plotly on PyPI and targets the doc-prod branch OR it targets the master branch
• [ X] If this PR modifies the first example in a page or adds a new one, it is a px example if at all possible
• [ X] Every new/modified example has a descriptive title and motivating sentence or paragraph
• [ X] Every new/modified example is independently runnable
• [ X] Every new/modified example is optimized for short line count and focuses on the Plotly/visualization-related aspects of the example rather than the computation required to produce the data being visualized
• [ X] Meaningful/relatable datasets are used for all new examples instead of randomly-generated data where possible
• [ N/A] The random seed is set if using randomly-generated data in new/modified examples
• [ N/A] New/modified remote datasets are loaded from https://plotly.github.io/datasets and added to https://github.com/plotly/datasets
• [ X] Large computations are avoided in the new/modified examples in favour of loading remote datasets that represent the output of such computations
• [ X] Imports are plotly.graph_objects as go / plotly.express as px / plotly.io as pio
• [ We need 2 DFs] Data frames are always called df
• [ X] fig = <something> call is high up in each new/modified example (either px.<something> or make_subplots or go.Figure)
• [ N/A] Liberal use is made of fig.add_* and fig.update_* rather than go.Figure(data=..., layout=...) in every new/modified example
• [ N/A] Specific adders and updaters like fig.add_shape and fig.update_xaxes are used instead of big fig.update_layout calls in every new/modified example
• [ No. The whole point is to illustrate an alternative to fig.show. We may want to integrate the result with the documentation in either a manual or automated way. Let me know if I should drop it in] fig.show() is at the end of each new/modified example
• [ X] plotly.plot() and plotly.iplot() are not used in any new/modified example
• [ X] Hex codes for colors are not used in any new/modified example in favour of these nice ones

I appreciate the Plotly community and hope this is helpful!

Code PR

• I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the codegen files and not generated files.
• I have added tests (if submitting a new feature or correcting a bug) or
  modified existing tests.
• For a new feature, I have added documentation examples in an existing or
  new tutorial notebook (please see the doc checklist as well).
• I have added a CHANGELOG entry if fixing/changing/adding anything substantial.
• For a new feature or a change in behaviour, I have updated the relevant docstrings in the code to describe the feature or behaviour (please see the doc checklist as well).

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