Monochrome uses built-in menu system supported by hugo itself. (Checkout document for more deatil.) By default, the menu called navbar is used to decide which item need to be rendered on the navbar.
So you can simply add the following code to your page front-matter, and the menu info will be handled by hugo out of the box. (simple method mentioned in document)
---
menu: navbar
---
Also, weighted value is supported out of the box, you can explicit assign the weight of the navbar entry, hugo will first sort the menu using weight, if there has two entries shared the same weight, the default order will be applied.