[Docs] Refresh iOS tutorial

[BalestraPatrick]

Recently, the ios-app project living in bazelbuild/examples was removed. This PR contains a few improvements:

• I updated our tutorial that shows how to create create a simple iOS app that builds with Bazel not to rely or point to the non-existing project.
• We now provide all the source code (one Swift file and one Info.plist) to generate a basic sample iOS app in this tutorial.
• The source code is now Swift instead of Objective-C.
• Simplified many sentences and commands, along with a few bug fixes required due to breaking changes in rules_xcodeproj (it now requires a @bazel_features explicit dependency).

I think it would be a good idea to move the tutorial to use bzlmod once Bazel 7 ships. For now, we can continue using the WORKSPACE for ease of compatibility with the default Bazel 6 behavior.

[brentleyjones]

Meta comment, @mattrobmattrob has a wonderful template for iOS apps: https://github.com/mattrobmattrob/bazel-ios-swiftui-template. I wonder if we can leverage it in any way (maybe just mentioning it)?

[BalestraPatrick]

@brentleyjones @mattrobmattrob I wasn't aware of that! Awesome resource, added a note about it.