When writing a noticeboard post
As you're writing a post on the noticeboard, you can add links to folders and files in the documents section by doing the following:
Click on the link icon in the toolbar:
In the Edit link window, you can (optionally) fill in the Text to display field, then either type in the Link to field to find the folder/file...
... or click on the folder icon to browse through your board's documents and select the folder/file that you want to link. This fills in the Text to display and Link to fields with the file/folder name and link.
From the documents section
You can also share folders and files from the documents section by opening the three-dot menu to the right-hand side of the file/folder and clicking Share on Noticeboard.
This creates a message on the noticeboard containing a link to the file/folder.
You can change the body of the post to write your own message about the file/folder you've shared, then choose who the email should be sent to, and hit Send to post the message.