(windows) deprecate Windows platform in documentation

[brody4hire]

Status

• pending formal vote

Platforms affected

Windows

Motivation and Context

see this discussion in dev@cordova.apache.org and apache/cordova-lib#852

Description

• add a note that Windows platform is now deprecated
• show "(deprecated)" in TOC title of Windows platform documentation

Testing

Visual inspection

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[purplecabbage]

I don't think this requires a formal vote, the numbers indicate this is a good idea.
I'll leave this open for a few days to gather thumbs up.

[brody4hire]

Thanks to @purplecabbage for merging these!