Add docs on "Create app flavors for Windows and Linux".

[kjxbyz]

Flutter 3.47 will be released next month. This version adds the ability to setup app flavors on Windows and Linux. Please add relevant documentation.

PRs:
flutter/flutter#187034
flutter/flutter#187029

[kjxbyz]

@sfshaza2 Sorry to bother you, is there any update on this issue?

[sfshaza2]

@kjxbyz, I have created a page but the bot would like more detail and this is way out of my area of expertise. I have asked the author for suggestions in the PR. (FYI, I was OOO for the last month.)