[doc] Add documentation generation with sphinx

[jmkerloch]

Add documentation with sphinx:

• [x] add conf.py to define sphinx option
• [x] move all .md documentation files to the documentation directory
• [x] create landing page documentation/index.md

[jmkerloch]

@lgrd @azarz I'm moving all documentation to the documentation directory. Without move I can't use the file in the table of content used by Sphinx.

[lgrd]

yes, it's ok. But you will have to change some links I think.