Add doc notes about using quote marks on Mac when installing optional et al

[kandersolar]

• Closes #xxxx
• I am familiar with the contributing guidelines
• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

pvlib defines a few sets of optional dependencies that can be installed with pip install pvlib[optional], pip install pvlib[test], etc. On zsh, the default shell on Mac, the brackets syntax is interpreted by the shell instead of being passed through to pip, so they must be escaped or wrapped in quotation marks like pip install "pvlib[optional]".

In #1167 we decided against documenting this since it's not specific to pvlib, but since it has come up again in the JOSS review (openjournals/joss-reviews#5994 (comment)) I think it's worth a small note in our docs.

This PR edits the installation and contributing docs pages to mention the correct command for Mac users.

[kandersolar]

@phoebe-p, do you think this PR will help new users not get confused?

[phoebe-p]

This looks good to me!

[kandersolar]

I don't think these changes will be controversial, so I'll go ahead and merge this PR. Thanks @phoebe-p for raising the issue!