Disable Jekyll to enable examples in Github pages

[rudihorn]

By default GitHub pages use Jekyll, which ignores all files starting with an underscore (https://help.github.com/articles/files-that-start-with-an-underscore-are-missing/).

This both means long times till the site appears on GitHub pages (as it all has to go through Jekyll) and the examples 404 (https://japaric.github.io/stm32f103xx-hal/stm32f103xx_hal/examples/_00_hello/index.html).

I've updated the after_success.sh script so it creates a file called .nojekyll so that Jekyll is disabled, which seems to solve the problem (https://rudihorn.github.io/stm32f103xx-hal/stm32f103xx_hal/examples/_00_hello/index.html).

The one issue I faced was that seemingly builds are disabled for the master branch, but only the master branch can produce the GitHub pages, so I have changed this to the staging branch? This might not be desired.