Add FAQ section to Portable Mode Docs

[nguyen-dows]

Add a FAQ section to the Portable Mode docs and address #665

[learn-build-service-prod]

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[DHowett]

heuristic feels like a very strange word to use here. This is more of a "practice".

[DHowett]

Honestly this doesn't explain why it doesn't work.

Suggested change

	Portable mode promises a terminal that is encapsulated in a separate folder (i.e. a flash drive). References to `ms-appdata` will not work in Portable mode however references `ms-appx` will.
	Portable mode offers a self-contained Terminal installation, where user data and application data are stored in the same place. As there is no separate user data folder, references to such folder (e.g. with `ms-appdata`) will not work.
	To refer to paths relative to the application install directory, use an `ms-appx:` URL.
	To refer to paths relative to the settings directory, use the environment variable `%WT_SETTINGS_DIR%`.