Simplify copy on Install page

[palewire]

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[palewire]

Once we settle this one, @choldgraf, I think I'm prepared to end my War on Cruft.

[choldgraf]

I think this is generally an improvement, but I'm worried that we're putting too much information in front of people we're assuming are total beginners. As a general rule, I think tutorials are best if they can be exactly followed from top to bottom with just copy-pasting. I'd rather we recommend a single option for people to type in, and then provide links for other options/more information if they want to dig deeper.

So, I've updated a few of the lines here to recommend the use of pip and moved mentions of conda/mamba/pipenv to asides. What do you think about that?

[palewire]

A made a bunch of trims in line with your suggestions. Have another look.

[choldgraf]

This looks good to me - I think it's much simpler, but for "total beginner install docs", I think that's a good thing :-)