[WIP] Sphinx-RTD

[wifasoi]

This PR will add the documentation for the VexRiscv project: DO NOT MERGE UNTIL:

• [ ] Add tag base versioning to sphinx
• [ ] Set up Travis CI job to push the docs in the gh-pages branch
• [ ] Link to meta-doc in the SpinalHDL repo
• [ ] Split Up the mono-page documentation
• [ ] Add gravitar

[wifasoi]

This is related to #47