docs: clarify broadcasting example title, subtitle, and summary paragraph

[mujju-212]

Summary

Addresses the first two checklist items from issue #564:

• Find a more appropriate title + subtitle, and write an informative short paragraph description at the top.
• Update the contents to make them easier to follow (separate follow-up PR)

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What changed

1. Title (sphinx-gallery heading)

Before	After
Broadcast functions across multi-dimensional data	Apply custom functions across movement data dimensions

The old title used the technical term "broadcast" without context. The new title immediately conveys what you are doing (applying custom functions) and where (across a movement dataset), which is more informative for users browsing the example gallery.

2. Subtitle (second line of docstring)

Before	After
Use the @make_broadcastable decorator to efficiently apply functions across any data dimension.	Use the ``@make_broadcastable`` decorator to run your own analysis functions across all individuals, keypoints, and time frames—without nested loops.

The new subtitle names the concrete axes (individuals, keypoints, time frames) that users typically iterate over, and explicitly mentions the problem it solves (nested loops), giving readers an immediate reason to care.

3. Summary section (introductory paragraph)

The old summary jumped straight to describing the decorator, without first stating the problem. The new paragraph follows the structure:

1. Problem — I have a function that works on a single (x, y) point; I want to apply it across the whole dataset.
2. Naive solution — nested for loops are verbose, error-prone, and tightly coupled to the dataset shape.
3. Solution — @make_broadcastable decorates your function once, so it accepts an entire DataArray, applies along the chosen dimension, and returns a new DataArray with coordinates intact.
4. Preview — this example uses a point-in-rectangle check as a case study.

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References

Closes the first checkbox of #564. The second checkbox (content improvements) will be addressed in a separate PR as suggested in the issue.

Checklist

• The code has been tested locally
• Tests have been added to cover all new functionality (N/A — documentation only)
• The documentation has been updated to reflect any changes
• The code has been formatted with pre-commit

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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