FLUID-5235: Add text about license to the footer of the docs.

[acheetham]

@michelled, I took the form of the attribution from CC's site: http://creativecommons.org/choose/results-one?license_code=by&jurisdiction=&version=3.0&lang=en

[jobara]

@acheetham and @michelled should we mention that the code snippets have a different licence. https://github.com/fluid-project/infusion/blob/master/Infusion-LICENSE.txt

[acheetham]

@michelled, @jobara, @colinbdclark: I've added the code license to the footer.

[CLAassistant]

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[jobara]

This PR is now out of date, given that it was filed 4.5 years ago it's held up pretty well. The main issue is that the license is now CC BY 4.0. I'm going to mothball this PR and we'll need to redo the work.

[jobara]

This has now been resolved in #310