User documentation

Description of change

Based on the wonderful work of @strangenoise.
Converted his work from markdown to reST format and documentation building with Sphinx.

• User documentation written. Go to docs/ and run either make html on Unix or the make.bat on Win.
• Added a 'video' directive to optimize GIFs loading.

⚠ For now the documentation is only local and have to be built until Stax goes Open. When this time will come, we will autobuild it with ReadTheDocs.

Closes #69 (closed)

How to test it

1. Install sphinx: pip install sphinx sphinx_rtd_theme
2. Go to docs/ and run either make html on Unix or the make.bat on Win.
3. Open the built index.html in a browser.

Checklist

Please check if your MR fulfills the following requirements:

• Unit tests have been added, if needed
• Documentation have been updated, if needed
• Build was run locally and any changes were pushed
• Lint has passed locally without error
• Package.py have been updated, if needed
• CHANGELOG.md have been updated, if needed
• Version numer has been tagged, if needed

FYI: @doubleailes @seblrd @olivier.hoareau @alefeve