Extend the introduction

Add to the introduction a short explanation of what the reserves are and why are important

docs/src/tutorial/reserves.md

@@ -6,6 +6,8 @@ This tutorial provides a step-by-step guide to include reserve requirements in a
 
 Welcome to our tutorial, where we will walk you through the process of adding a new reserve node in SpineOpt using Spine Toolbox. To get the most out of this tutorial, we suggest first completing the Simple System tutorial, which can be found [here](https://spine-tools.github.io/SpineOpt.jl/latest/tutorial/simple_system/).
 
+Reserves refer to the capacity or energy that is kept as a backup to ensure the power system's reliability. This reserve capacity can be brought online automatically or manually in the event of unforeseen system disruptions such as generation failure, transmission line failure, or a sudden increase in demand. Operating reserves are essential to ensure that there is always enough generation capacity available to meet demand, even in the face of unforeseen system disruptions.
+
 ### Model assumptions
 
 - The reserve node has a requirement of 20MW for upwards reserve