Add germination tutorial to gallery

[Stylopidae1793]

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these changes.

Type of update
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• Bug fix
• New feature or feature enhancement
• Update to documentation
• Work in progress

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For the reviewer
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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

[codecov]

Codecov Report

Merging #1359 (82d1b9b) into main (5fe40c7) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1359   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          155       155           
  Lines         6840      6840           
=========================================
  Hits          6840      6840           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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