Update Sphinx config for favicon

[tcmetzger]

Thanks for using Sphinx Favicon in your project! I just released version 1.0 of the extension, which brings one breaking change: to better conform with Python standards, we changed the module name to sphinx_favicon (instead of sphinx-favicon). This means you'll have to update the name in the extensions list of your conf.py file (https://github.com/fluiddyn/fluidfoam/blob/master/doc/conf.py#L62) before you can use version 1.0. Otherwise, your existing configuration should continue to work!

[CyrilleBonamy]

Thank you for your comment.

Done #51