Improve installation instructions

[adswa]

• Start more general with the installation; note that python, pip, git, git-annex and datalad are different components
• More verbosity on existing instructions; info on how to install homebrew or do a pip install --user
• Restructure hierarchy: start with Linux, then OSX, pip, Windows
• add installation instruction for Windows without and with WSL2. I've only tested this on a very old computer and it seems to work, but there are no guarantees for anything. In any way, its better to have something than nothing, I suppose. What is an absolute life-saver is Neurodebian.

Still-TODOs:

• How to install Git-annex without administrative power
• mabye tell everyone who is not on Windows 10 that thats a problem?

[adswa]

(Note that there is a keyserver issue on WSL which I'm trying to mitigate by having users curl and apt-key add the Neurodebian repository key)

[adswa]

Todays' discussion showed that we should try it out on WSL2 and provide installation instructions for this as well. From my point of view, there are improvements in this PR to the general feel and content of the installation instruction, and even though Windows installation instructions are yet incomplete, its better than having none.

[mih]

This sounds scary, but it only refers to the terminal window with datalad inside, no?

[adswa]

Thx, @mih!

[mih]

Pushed minor changes and merged master to get into mergeable state. Ready to go once tests have passed.

[adswa]

Nice, thx!