Can I add related documents (instructions) into a particular skill?

Yes, you can attach a document as below:

1. Navigate to "Skills" in the Skill Matrix Submenu:  
2. If you are Editing an existing Skill, select the Blue Edit Pencil to the right of the Skill where you wish to upload documents. If you chose to add a new Skill, review how and where to add documents here. 
3. Scroll down to the "Qualification Requirements" Here you can select internal TeamGuru documents including the Documents module for any instructions, SOPs, or relevant qualification material. Multiple documents can be saved to each qualification requirement.