You can also just enter id of any item, folder or JIRA ticket inside rich text boxes (e.g. the description of an item). These will be automatically converted to hyperlinks allowing you to navigate quickly inside the project.
Hyperlink visible after saving:
You can also type the item id followed by a ! sign and it will be rendered as hyperlink and the title of this item right after it, e.g. DOC-1! will be shown as DOC-1 document 1 title.
There is a menu in the editor that eases introduction of these links, allowing you also to enter links to items in other projects:
New editor (from 2.2):