Update PlantCV installation instructions and conda environment file to use only the conda-forge channel

[nfahlgren]

Describe your changes
Usage of the Anaconda defaults channel is not free for organizations with >200 employees. PlantCV only uses the conda-forge channel, this PR updates the documentation to strongly encourage use of installation of conda using Miniforge, which configures conda to use only the conda-forge channel by default. This helps prevent users from inadvertently using the defaults channel.

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

[deepsource-io]

Here's the code health analysis summary for commits d874246..9373a17. 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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