HTTP client docs

[ChristophWurst]

Apparently we do not have this covered with our available dev docs, but it's quite important so there should be some coverage with examples of how to use \OCP\Http\Client\IClientService.

[ChristophWurst]

Small update: discussing this topic with a few people made us realize that our own abstraction isn't such a great idea by current standards anymore and we'll have a more solid HTTP client if we "just" offered a PSR HTTP client. Nevertheless this needs docs.