Add notes about purging static content from iOS/macOS apps.

[freakboy3742]

Adds platform notes on how to purge binary content from apps on Apple platforms, to avoid issues with iOS App Store validation and issues with binary merging on macOS.

Fixes #2161

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[mhsmith]

Can we take this opportunity to eliminate all the duplication between the app and xcode pages? They're already virtually identical. And including the same content on both pages will cause the user to waste time visually comparing them for differences.

We could move the common content to the top-level macOS page, and link to it from the sub-pages. This would also make the actual differences much more obvious.

[freakboy3742]

I guess you're right - and the same is true for the Windows app and VS pages. There's already some inclusion blocks, but I suppose almost entire page except for the format description is a candidate for being an inclusion... which means we should probably just cut out the middle-man.