Upgrade to Apple Silicon on CI, simplify Apple Silicon installation instructions

[nilsvu]

Proposed changes

Upgrade the macOS CI test to run on Apple Silicon. Also simplify Apple Silicon installation instructions a bit.

Upgrade instructions

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments

[nilsdeppe]

Thank you!

[knelli2]

MacOS spack installation of pybind11 isn't happy

[knelli2]

The SetupPybind11.cmake isn't finding the boostrapped pybind11. Maybe we need another hint in cmake?

[nilsvu]

@geoffrey4444 FYI. Still needs #5956 to be merged first.

[geoffrey4444]

I'm excited about adding Apple Silicon to the CI...thanks for doing this!

I understand there are complexities to #5956 , so if that looks like it won't get merged for a while, maybe consider just implementing the slightly longer instructions in a separate PR, so we can start automatically checking Apple Silicon? We can always improve things later, but we have enough Apple Silicon users that it would be nice to get the CI up and running. But if $5956 is going to get merged soon-ish, maybe a separate PR isn't worth it.

For those who prefer manual installation (I'm probably one of them, at least for now), maybe instead of deleting the longer instructions, move them down to a "manually install dependencies" section?

[nilsvu]

@geoffrey4444 ok I removed the dependence on #5956 for now. Once it's merged we can simplify the instructions further, but I agree that it would be good to keep an (optional) "install manually" step.

[nilsvu]

@nilsdeppe @geoffrey4444 @knelli2 I rebased and updated this. Please, could whoever of you is interested in this take another look so we can get this in? It would be helpful to have an ARM macOS build on CI.

[knelli2]

Changes look ok to me, but I don't have a mac to test.

[knelli2]

I guess it's up to @geoffrey4444 if he wants the docs default to install extra dependencies in people's build directories

[geoffrey4444]

I think fetching missing dependencies is fine

[geoffrey4444]

LGTM, but I notice it has conflicts that must be resolved first

[nilsvu]

Rebased!

[knelli2]

Tolerances on the partial derivative test still seem to not be adequate.

[nilsdeppe]

No need for my approval on this. I was pinged in the past, but have nothing to add.

[nilsvu]

Tolerances seem to work now @knelli2