Install: Rework macOS installation

[adswa]

• base installation on brew
• move pip installation more into paragraph on pip

[tamaracha]

I think you can merge this PR, so the central part is pointed out. In the next time, I will probably address more topics, because we are planning to integrate datalad in our workflows at CRC 135.

[adswa]

Thanks much, @tamaracha!
I'm trying to keep track of contributions via the all-contributors spec and zenodo, and the main datalad repo tries to do so, too. Please feel free to add yourself (or tell me to add you) to .zenodo.json of the handbook and the main datalad repo (https://github.com/datalad/datalad/blob/master/.zenodo.json) - this would add you as a co-author to the zenodo record of the handbook and datalad, if you want.
Hey @all-contributors please add @tamaracha for review, infra.

[allcontributors]

@adswa

I could not determine your intention.

Basic usage: @all-contributors please add @Someone for code, doc and infra

For other usages see the documentation

[adswa]

Hey @all-contributors please add @tamaracha for review, infra

[allcontributors]

@adswa

I've put up a pull request to add @tamaracha! 🎉