doc: Add `BackgroundTaskThread` python example

[riskydissonance]

To allow python plugins to do work not on the UI thread.

Also allows them to call update_analysis_and_wait().

Tested with a basic plugin.json as a full plugin and works.

[CLAassistant]

All committers have signed the CLA.

[CouleeApps]

Nice basic example and works as expected. Thanks.