(As a side note, Pinault is married to actress Salma Hayek, not that being a billionaire and CEO of an international luxury group whose holdings includes Gucci and Saint Laurent isn't noteworthy enough. (As a bonus, he's also president of the French holding company which owns the fine arts auction house Christie's.)
The French government's fundraising site hasn't hasn't been set up yet, but at the moment there are two other venues for donating. And while I suspect they will meet their goals with donations pouring in from around the world, and other major donations from wealthy French individuals and those across the globe, if you're interested in donating, these are the two leading charities which news sources have pointed to --
Friends of Notre-Dame has offices in both France and the U.S., It previously established itself as a 501c3 charity when the restoration had earlier begun that was in operation when the fire broke out, and had been the primary organization raising money for the restoration already under way. You can reach their website here, although the site seemed to be down much of last evening, perhaps from overload, though I did finally get through.
Fondation du Patrimoine is a French nonprofit dedicated to the preservation of historic, cultural sites throughout France. It has established a special Note Dame rebuilding fund. You can get to their website here -- just know that the site is in French. However, most browsers should pop-up a window asking if you want the page translated. If you don't get that, the boxes asking for information should be pretty clear. Also, it asks for donations in Euros, so know that one Euro equals $1.13,
By the way, in addition to the foundation structure being saved, along with much artwork, relics and icons inside, it was wonderful to find out that it appears the famous North Rose Window was not lost.