Add a page in the user's guide for Saving and Loading sessions

Saving and Loading sessions in IGB is, or should be , a big feature of IGB, especially since there are default buttons for these actions in the IGB toolbar.

But it seems that there is no page in the user's guide for this feature.

Instead of letting users get confused about any limitations this feature may have, we should outline how to use the feature and what it can do in the IGB user's guide.