* alter docstring of @api.one to mark it as deprecated for 9.0,
recommend using @api.multi instead
- deprecation notes were not correctly styled, add styling
matching "warning" alerts
* move @api.one down the doc page to deemphasize it
* fix "backend" tutorial to remove all instances of ``@api.one``
closes#8527
Pretty much completely rewritten theme with custom HTML translator and a
few parts of the old theme extracted to their own extensions.
Banner images thought not to be that huge after all, and not worth the
hassle of them living in a different repository.
co-authored with @stefanorigano
Xavier Morel