Re-order Pixi commands and update Mac instructions in index.md

[psommerfeld]

Pixi build commands were listed before the git clone commands which doesn't make sense. Put the commands in more proper sequence as they would be executed. Before, Pixi was described as a good way to compile, then later only CMake is mentioned. Broke up Build section to provide both Pixi and CMake build methods. Mentioned 'brew install pre-commit' as Mac method to install pre-commit

[chennes]

Thanks for the PR, @psommerfeld -- I'll ask our resident Pixi expert @oursland to give the changes a look.

[maxwxyz]

@psommerfeld ping for feedback

[psommerfeld]

I somehow missed the change request. I never got Pixi to work and it's been too long since I looked at it to remember why I found it confusing. So, sorry, my input wouldn't be of value anymore.

[hyarion]

I somehow missed the change request. I never got Pixi to work and it's been too long since I looked at it to remember why I found it confusing. So, sorry, my input wouldn't be of value anymore.

Thanks for lettings us know, and sorry to hear that you couldn't get it to work. If nothing else you've highlighted that this document needs further work, so thanks for pointing that out!