Add docs - merge first ?

[HaleySchuhl]

Describe your changes
Create structure to build a documentation page on ReadTheDocs with a few basic pages to start.

Type of update
Is this a:

• Update to documentation

Associated issues

• closes Add Documentation  #7

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

[deepsource-io]

Here's the code health analysis summary for commits 32922ef..11f0983. 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%

---

💡 If you’re a repository administrator, you can configure the quality gates from the settings.

[nfahlgren]

There are links to files that do not exist, these are easy to flag by building the documentation locally using mkdocs serve --theme readthedocs