Include cache handling in documentation on audb load process

[hagenw]

At https://audeering.github.io/audb/overview.html#load we discuss the single steps performed when a database is loaded:

But as pointed out correctly at #434 (comment), when the identical media file or table exists already in the cache of another version of the database, it is copied from there instead of downloaded from the server. This is a minor detail, but can save a lot of bandwidth as most of the media files tend not to change between versions of a database.

Would be nice to find a way to integrate it to the figure (or at least inot the text) without making it a lot more complicated.