add animation gifs for new rack-mount systems, also update rbuild and the main readme to reflect these changes

rack-mount/enclosed-box/README.md

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+# Enclosed Box Mounting System
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+![demo](./animate.gif)
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+Please see [entry.scad](./entry.scad) for config and building. Make sure the correct rack profile is configured in [rackFrame.scad](../../config/rackFrame.scad).
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+- This system comprises three parts: two side rails, and a front plate.
+- This system is useful for mounting heavier box-shaped things. It works by first sliding in the box inside the side rails (which secures `x`, `z`, and one half of the `y` axis), and then mounting the front plate along the main rails.
+- Side rails will most likely require supports to print.
+- For wider boxes (nearing the `maxUnitWidth` of the rack), you will have to set `recessSideRail` to true. This is so the side rails will not collide with the main rack rails.