Make documentation navigation more intuitive

[tbittar]

Several remarks have been made on the docs :

• The performance graph page is hard to find from the home page for a new user
• The navigation tabs on the Antares-Xpansion home page are on the top of the page and they can be confused with the table of contents of the page that are on the right hand side of the page

To answer to these remarks :

• We can move the performance graph page on the home page
• We need to think about the organization of the tabs on the home page (put them on the left ?)

[JasonMarechal25]

Is it still relevant ?