Add an mBuild docs example of changing an existing box's dimensions and angles for the new mbuild.Box's structure.

[bc118]

Since we changed the mbuild.Box's structure, the individual box dimensions and angles are now immutable. This change may be confusing to some users when trying to change the box's existing box's dimensions and angles. We should make an example in the mBuild docs showing the new way to change the mbuild.Box's box's dimensions and angles as a whole, as opposed to the old way, in which they could all be changed individually.