Rework documentation completely

[mwestphal]

• Remove previous doc
• Create a pure, github-compatible markdown based documentation: https://github.com/mwestphal/f3d/tree/jekyll_doc
• Add a just-the-docs jekyll based deployement: https://mwestphal.github.io/f3d/
• Image and videos are hosted on github freely, they are listed here: Rework documentation #48

TODO on merging this:

• : Change the github pages deployement setup to use jekyll on master
• : delete the page branch
• : Store media as lfs in a dedicated repository: https://github.com/f3d-app/f3d-media

[CharlesGueunet]

Ideas:

• Add a menu / summary
• Add a changelog entry with the changelog of the last relase
• Add an icone in the tab of the browser
• Use the space aside from the big f3d icon at the top

[Meakk]

Comments on the website:

• The link "F3D on github" on the top right isn't great. Can you maybe add a github logo and a better text like "Explore code source on GitHub"
• The /fɛd/ text does not appear correctly on my mobile
• Can we replace the animated gif on the home page with a video?
• For the cpp examples, we should add #include <f3d/xxx.h>