Add draft for Database page

Database.md

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+**_This page is in an early state and not yet completed or reviewed!_**
+
+This pages gives some information on the internal app database. In short, you learn about the [database structure](#database-structure) and [how to modify it](#modifying-the-database-structure) as well as how to [access the stored data](#access-data).
+
+## Database structure
+NewPipe is a streaming frontend. For this reason, storing information on `streams` is the database's core functionality.  
+Basically, stream information is used for feeds, playlists and watch history.
+
+![NewPipe's database structure](https://user-images.githubusercontent.com/17365767/141841352-eefcd324-abfe-480b-9626-17db21f6c9de.png)
+
+### Modifying the database structure
+When modifying the database structure, the [database version needs to be increased](https://github.com/TeamNewPipe/NewPipe/blob/dev/app/src/main/java/org/schabi/newpipe/database/AppDatabase.java#L41).  
+You also need to create a migration to update the structure on installations which run an old database version. This is done in the [`Migrations`](https://github.com/TeamNewPipe/NewPipe/blob/dev/app/src/main/java/org/schabi/newpipe/database/Migrations.java) class. After creating the migration, it needs to be [registered with the database](https://github.com/TeamNewPipe/NewPipe/blob/dev/app/src/main/java/org/schabi/newpipe/NewPipeDatabase.java#L26) to be executed if necessary.
+
+## Access data
+Add some info on the DAO pattern here; AppDatabase and NewPipeDatabase