Update installation instructions to newer python version and give explanations

[campbell-reid-displayr]

Motivation

I was working through using mmsegmentation and some of the steps did not work. Trial and error led me to being able to use it so I thought I'd contribute back what works now.

Modification

Updated some instructions and package versions to what is working today.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMDet3D.
4. The documentation has been modified accordingly, like docstring or example tutorials.

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