Document app patching

Create a documentation article for the app patching process. Include a link to the tabris.app.installPatch API reference.

[ralfstx]

@tbuschto We already have an article on app patching that states

This feature is considered provisional and the API described here may be adjusted in a future release.

I think unless we decide to remove this feature, we should include this method in the documentation and mark it as provisional.

Having that said, I also consider the feature very dangerous and given the faster turnaround times in the app stores, it might not be as critical anymore. However, that would be the topic of another issue.

[cookieguru]

@mpost No, I was just curious about the reasoning behind the change (e.g. technical limitation). I think the addition of fs will solve some of the issues that patching solved, with the exception of executable code. But replacing code was a potentially risky proposition.