Use petridish theme for documentation site

After discussing with @mtrekels @macSands @LauraAbr: this PR switches the Jekyll theme from just-the-docs to petridish

This makes it easier to:

Customize colours: the B-Cubed colours are now used for headers and links in _config.yml
Control navigation: rather than controlling it per page (defining parents, childs, nav_order) and having pages appear automatically, navigation is defined in a single file _data/navigation.yml
Control footer (with e.g. funding information)
Have in page navigation, just-the-docs requires this to be verbosely defined for each page and it only appears on the top, now it is set by default and appears on the side
Apply style with bootstrap classes
Invite people to collaborate with the edit this page link at the bottom, which opens the corresponding markdown page on Github
Extend with own layouts. I created a tutorial.html one that now displays the author + source

Downsides:

No side-wide search, but I hope I can implement that as part of petridish: https://github.com/peterdesmet/petridish/issues/98
Navigation goes 2 deep only. It is possible to have custom navigation for sections, but for now I think it's better that deep tutorials are manually linked from their parent.

Also addresses

Fix #15 Fix #6 Fix #5 Fix #4

Before:

The-README-file-B-Cubed-documentation-and-tutorials (1)

After:

The-README-file-B-Cubed-documentation-and-tutorials