Place to pin notes?

Is there a way to make a corkboard for notes, or a way to pin notes to the main manuscript board?

Yes, there is a checkbox in the Inspector (click the blue ‘i’ button on the right-hand side of the toolbar) called “Include in Compile”. If you turn that off, that particular item will not be included in the book, nor will its words be counted toward the total. You can use this to intersperse notes amongst manuscript text easily.

If you want to get fancy, you might also want to look into making a document template for this. If you started from a novel project, then you might already have one where character and location sheets are. Anything you put into that Blue T folder will become a template. You could create a document there with the checkbox already turned off and with a custom icon so it stands out. Then you can easily create a new note card with the Project/New from Template/ sub-menu.