Add docs that explain 3-way merge editor

[isidorn]

We got countless feedback that merge editor is confusing to use. We need to write docs to explain how to use the merge editor.
I suggest that we update the current docs for viewing conflicts and add a section just for the merge editor https://code.visualstudio.com/docs/editor/versioncontrol#_merge-conflicts

From docs we can also link out to a video that @burkeholland is working on