TODO for website

[comane]

This issue is to gather problems / improvements that are needed for the website of colibri to be ready.

• Index and module index seem unused.
• Search mode does not seem to work properly (compare it to NNPDFs docs for instance), is probably related to the above
• review of the installation instructions, remove redundancies and enhance it once we have a pip package
• Change "Colibri Team" to "Colibri Collaboration" and "Colibri has been developed by the following people" to "The Colibri collaboration currently consists of ..."
• Add contributing guidelines
• Change the description of colibri: "Colibri is a modern, high-performance framework for fast and flexible PDF fitting. It is a reportengine app to perform PDF fits using arbitrary parametrisations" to something without modern and high-performance since this attributes might change in time...
• Add citation policy (also to readme)