Add grayscale tutorial and replace old NIR

[HaleySchuhl]

Describe your changes
Updates the tutorial gallery to include a grayscale analysis example. This might be the simplest workflow a person could start with, and it replaces the old NIR tutorial.

Type of update
Is this a:

• Update to documentation

Associated issues
#1293

Additional context
Add any other context about the problem here.

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

[codecov]

Codecov Report

Merging #1300 (2701657) into release-4.0 (2a85248) will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           release-4.0     #1300   +/-   ##
=============================================
  Coverage       100.00%   100.00%           
=============================================
  Files              157       157           
  Lines             6757      6757           
=============================================
  Hits              6757      6757           

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

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