Website scaffold

[diegoferigo]

This PR initializes the empty website https://robotology.github.io/gym-ignition. It uses:

• Sphinx as main platform.
• Sphinx MultiVersion to handle multiple versions. Only master, devel, and v* tags are matched.

There are two new targets:

• doxygen build the Doxygen documentation
• sphinx builds the website and generates the folder <build>/html that can be deployed

The integration between sphinx and doxygen is not yet there and it will be done in another PR. I managed to make Sphinx display also the C++ documentation using breathe, inspired by this amazing tutorial, but I will populate APIs content in another PR.

At the moment I pushed the content manually. Sooner or later I will implement a new Workflow to automatize the process.

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Note: there is a similar contrib extension to build a multi-version website, but it's currently bugged (sphinx-contrib/sphinxcontrib-versioning#78). Though, the SMV extension works quite well.

[traversaro]

I am not fully convinced by the Gym-Ignition name with both capital letters and dash -, but that is just personal taste.

[diegoferigo]

It is the name we used in our publication. Though, for the website we can change it and make it nicer. The two options are:

• Gym Ignition
• gym-ignition

[traversaro]

I personally like gym-ignition because it is consistent with GitHub repo, pip package, package in any possible package manager in the world, but that is personal taste.

[diegoferigo]

And probably that would also work better for the indexing of search engines. Agreed.