Extend the 'windows-app' page from common-docs

[ganicke]

Change the flat version of windows-app.md to extend the base version from pxt/common-docs.

NOTE: not sure if this requires a bump of pxt to include the updated page from pxt/common-docs. I had to npm link ..\pxt to get the extend to work.

Closes #4159
RE: #4184, microsoft/pxt#8434

[ganicke]

@livcheerful - what should we do with this one-off troubleshooting page? - https://makecode.microbit.org/device/windows-app/troubleshoot

[livcheerful]

oh hurrrmmmm I would do the alert thing at the top and say check out the deprecate page, but still leave this up?

[abchatra]

@ganicke any update here?

[ganicke]

This depends on pxt 7.2.6 or better.