Create API reference in docs listing all possible annotations available.

[wtrocki]

Annotations are hard to work (as they require reference)
Our docs do not have a reference for those (they are spread across different topics and hard to find.

Adding new annotation brings mental challenge (where to add this)

[machi1990]

/cc @ssd71 looks like #1301 got merged 👍

[machi1990]

We have this https://github.com/aerogear/graphback/blob/master/docs/metadata.md that's a step closer to this goal. Also we this issue #1526 to work on docs generally. Can this be closed?
/cc @craicoverflow @wtrocki

[craicoverflow]

Yep, this can be closed thanks to the proactive work by @ssd71