updating the getting started page

[annacasto]

Describe your changes
Made some changes to the Getting Started section of index.md to increase readability and to create a list of documentation pages to get started on. I hoped this would give new users a place to find the most helpful documentation pages for going through the steps of installation, building a workflow, and running the workflow in parallel.

Type of update
Is this a:

• Update to documentation

Associated issues
Reference associated issue numbers. Does this pull request close any issues?

Additional context

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 c6105a7..86b3058. 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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