Custom response types docs and video streaming example

[michalpokusa]

⭐ Added:

• Docs about subclassing Response to make custom response types
• Example showing custom XMixedReplaceResponse for streaming video from camera which resolves add support for "multipart/x-mixed-replace" (for streaming video) #95

🪛Fixes:

• Typo in multiple servers example

The example should work out of the box on MEMENTO.

The main reasoning behind not adding this response type as part of library itself is that it is already quite big, and the use case is not as common as e.g. Websockets.