Rework Installation Instructions

[Bengt]

Patch description

• Add some structure to clearly distinguish texts regarding the operating system, Python interpreter, and virtual environment
• Reformat end user and developer installation headings to use ATX-style headers for cleaner use of markdown
• Extend instructions to use a virtual environment as recommended in the text

Testing steps

• Run the instructions on a supported operating system and observe that they work flawlessly as-is.

Other information

These fixes are largely stylistic, but the instructions are now more complete and actually work.

[Bengt]

@klshuster Thanks for having a look at this. I would be glad to contribute. I was in need of better installation instructions, and I hope I can help the next person in my position.

[klshuster]

Could you please rebase onto the main branch? then I think we can go ahead and merge this

[github-actions[bot]]

This PR has not had activity in 30 days. Closing due to staleness.