Create plot_extraction.md

[dhiraj-ms]

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• Bug fix
• New feature or feature enhancement
• Update to documentation
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• PR functionality reviewed in a Jupyter Notebook
• All tests pass
• Test coverage remains 100%
• Documentation tested
• New documentation pages added to plantcv/mkdocs.yml
• Changes to function input/output signatures added to updating.md
• Code reviewed
• PR approved

[deepsource-io]

Here's the code health analysis summary for commits 61c76c0..300409e. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Python	❌ Failure	❗ 7 occurences introduced	View Check ↗
Test coverage	❌ Failure	❗ 1 occurence introduced	View Check ↗

Code Coverage Report

Metric	Aggregate	Python
Branch Coverage	100%	100%
Composite Coverage	56.8% (down 43.2% from main)	56.8% (down 43.2% from main)
Line Coverage	56.8% (down 43.2% from main)	56.8% (down 43.2% from main)
New Branch Coverage	100%	100%
New Composite Coverage	0%	0%
New Line Coverage	0%	0%

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[HaleySchuhl]

This is super minor, but having the filename closely matching the function name will make things more searchable as this repository gets larger in the future. Therefore, I suggest changing the filename to "extract_plot.md" here and the function file to "extract_plot.py".

Generally, this PR is adding a few functions all under the aim of plot extraction, but do we only expect an end user to call the function "write_shapefile"? If so, we can consider making the syntax of the other functions similar to that used in "helper functions" in the main namespace (e.g. _calculate_grid is a helper function of auto_grid inside roi_methods.py).