Add a section to explain displaying chords to the stage display

[Johnthan]

Windows 10 Pro
OpenLP version 2.4.6

Displaying chords on the stage display has come up a few times in the forums and there are great sites to help. Putting it in the manual may help users to use this feature.

This is one of the discussions:
https://forums.openlp.org/discussion/4266/functions-to-add-volume-keys-change-slides-stage-view-can-also-show-chords#latest

This process includes:
Setup:
1. creating two new formatting tags
2. creating one or two modified stage displays
On-Going:
1. using the text to OpenLP HTML to structure new songs with chords
2. modifying existing songs to add chords

Thanks to Everyone that made this possible!