SCM - Add accessibility help dialog

[meganrogge]

cc @jooyoungseo, @lszomoru

JooYoung mentioned there are some tab stops that aren't necessary here, making navigating it cumbersome.

He also indicated that we should give users an overview of this view and inform them of useful actions that can be taken.

[meganrogge]

If possible, the changes should be tab navigable (right now a user has to use down arrow to reach them)

[jooyoungseo]

Another related issue is that there is no command that allows users to directly focus in the change tree view. I suggest the following command to be added: 1. SCM: Focus in change view 2. SCM: Focus in staged view

[meganrogge]

#210255