[FIX] doc: guides section was removed, move CDN to reference
[FIX] ir_qweb: css asset: use which to find lessc and sass

Also, use lessc.cmd instead of lessc on Windows. It allows to run
lessc command with only nodejs and the less module (no cygwin).
[IMP]account : due date fill on change of payment term base on current date or invoice date,
[IMP]account_accountant, account_voucher: payment_option field are inherit in full accounting, and remove invisible attribute of payment_option and writeoff_amount