add material theme to mkdoc.yml and add copy button

[HaleySchuhl]

Describe your changes
Add copy button to code blocks with the mkdocs extension called material.

Type of update
Is this a:

• Update to documentation

Associated issues

• closes Add copy button to code blocks within documentation #1805

Additional context

• To build locally in your plantcv conda environment, add pip install mkdocs-material

For the reviewer
See this page for instructions on how to review the pull request.

• 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 5903d34..0414841. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Python	✅ Success		View Check ↗
Test coverage	✅ Success		View Check ↗

Code Coverage Report

Metric	Aggregate	Python
Branch Coverage	100%	100%
Composite Coverage	100%	100%
Line Coverage	100%	100%

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