Tanggung jawab notaris dalam memastikan keabsahan perjanjian pengikatan jual beli dan pembuatan surat kuasa membebankan hak tanggungan dalam transaksi pre-project selling = The responsibility of a notary in ensuring the validity of a conditional sale and purchase agreement and the grant of the power of attorney to create encumbrance (surat kuasa membebankan hak tanggungan / SKMHT) in pre project selling transactions