better theme for gitlab pages

Hi @embray @fjay @thsanche @pjobic !

What do you think about changing the template of the gitlab pages which, I found, make them particularly difficult to read.

I propose 3 proposition in addition to the actual one:

Current 1️⃣	Sphinx RTD 2️⃣	AGOGO 3️⃣	Bootstrap & Readable 4️⃣

You can vote by choosing the different numbers. I'll make a merge request so you can try. You can also propose something else.

The bootstrap one have different sub-themes (see link in table) and appears to be more customizable.