Add extra table of contents links

[nfahlgren]

Describe your changes
Functions are organized by common purpose or data type. For example, all analysis functions are organized in plantcv.analyze. The documentation TOC follows this logic, but in some cases it is useful to also link to a function based on a shared set of functionality, such as all functions related to Hyperspectral image analysis.

This PR adds duplicate links to the TOC of the documents so that functions can be found both in their actual submodule but also in relevant groupings based on function. In particular, duplicate links are added to the hyperspectral and photosynthesis sections, and a new Inputs and Outputs section is added at the top to make the basic data handling functions easier to find.

Type of update
Is this a: Update to documentation

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 #1326 (831e043) into release-4.0 (5074924) will not change coverage.
The diff coverage is n/a.

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##           release-4.0     #1326   +/-   ##
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  Coverage       100.00%   100.00%           
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  Files              157       157           
  Lines             6777      6777           
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  Hits              6777      6777           

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

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