Move tutorials text into markdown inside this repository

1. https://coaxion.net/blog/2018/01/how-to-write-gstreamer-elements-in-rust-part-1-a-video-filter-for-converting-rgb-to-grayscale/
2. https://coaxion.net/blog/2018/02/how-to-write-gstreamer-elements-in-rust-part-2-a-raw-audio-sine-wave-source/

These two here. They also need to be updated a bit for the new APIs and new locations of the code.