add database doc in dia

[theScrabi]

This will add a databasedocumentation created using dia. It is stored in the assets folder, and is ment to be used in the wiki/documentation.

@TobiGr it would be nice if you could put the svg to it somewhere usefull :).

[TobiGr]

👍 Will do this next week after I finished my last exam.

[theScrabi]

Uff two tables have the name search_history :/. Well the one pointing to streams should be called watch_history. (I think).

[TobiGr]

Yes, you're right

[TobiGr]

it would be nice if you could put the svg to it somewhere usefull :).

@theScrabi I'd suggest to put it into the wiki and
a) create a new chapter in the documentation explaining which steps should be followed before/when implementing a new feature or
b) create a new chapter about the NewPipe's. (btw. what about the giga / downloads database which is located here?)