FIX] website: link enable google maps required billing

Since 11 June 2018, enabling Billing on your Google Project used for
showing static maps is required.

So you would follow the only direction of Odoo "Create a Google Project
and Get a Key" and nothing would work since you still need to enable
billing.

With this changeset, we add a link to the Google page explaining maps
billing costs, current free usage tier and so on. And on this page the
"Get started" button will enable billing if needed.

There is another page with deeper explanation but it is very technical
and currently only available in english:

https://developers.google.com/maps/gmp-get-started



note: also improve text of has_google_maps option

opw-2210346
closes #47227

closes odoo/odoo#47599

X-original-commit: 6035bba8b679620dcbfc7ba620aafa13009ff616
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>