update contribution guide

[maliagehan]

add tutorial contribution guide and table of contents

Describe your changes
Updates contribution guide to add a table of contents because it's long and also adds a contribution guide for adding tutorials.

Type of update
Is this a:

• Update to documentation

Associated issues
#1299

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

[HaleySchuhl]

Thanks for adding this Malia!

I think we could add some clarification to the part about adding tutorials. I found that the most straightforward way to clone the tutorial template structure into a new repo is by making a completely empty repository, and then using this option on GitHub.com, providing them with the link to plantcv-tutorial-template. It's the option at the bottom of the page and looks like this..

[codecov]

Codecov Report

Merging #1320 (08a610f) into release-4.0 (18e4b6d) will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           release-4.0     #1320   +/-   ##
=============================================
  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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