Clarify macOS version support and minor style tweaks

[timsneath]

Clarify that container is only supported on macOS 26, and remove language that implies that it's also designed for macOS 15.

This change also includes a couple of minor Apple Style Guide changes, and links to the OCI container image format for clarity.

[petr-ujezdsky]

So.. Can we use container on macOS 15 or not? It used to work, with some limitations described in https://github.com/apple/container/blob/main/docs/technical-overview.md

I was also suggesting to improve tutorials with respect to these limitations (common pitfalls) for users using macOS 15, see #272 (comment)

I see it still available via brew for sequoia and later
https://github.com/Homebrew/homebrew-cask/blob/HEAD/Casks/c/container.rb#L16

[timsneath]

Hi Petr, we haven't made changes to explicitly block installation on macOS 15, but we built Containerization for macOS 26. We don't test it on older versions or intend to backport fixes to that release, so we want to clarify that Containerization is unsupported on that version.

[petr-ujezdsky]

Ok, thanks for that :) This PR is about "Clarifying supported macOS version" but the change made that information actually more opaque IMO. All I am saying is, that the current wording

We do not support older versions of macOS

could be a bit misleading. At first glance it would simply mean I can not install it on any other macOS than 26. Which is not true.

"Feature not supported" means a piece of hardware, software, or a service does not understand or cannot use a specific function, file type, or feature.

As a newcomer I was very happy to see macOS 15 mentioned explicitly in top-level README with some additional info about caveats. I suggest to be more explicit on which OS can run this (15 and 26) and which OS you are accepting bug reports and feature requests (26 only).