[docs] use docsy's new readfile shortcode

[polarweasel]

This is a temporary fix until Docsy 0.5.0 lands.

@netlify /preview/tabpane-include/

[polarweasel]

Hey @ddhodge I've added Docsy 0.5.0's readfile shortcode here. Want to play around and see if it's fixed the inclusion problem with tabpanes?

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[polarweasel]

As of fc65836, tabpane inclusions render now, but the behaviour of the tabs and their content is weird. One pane's tabs will switch, but the other pane's tabs won't. I think it's the first one you click. But then clicking the second pane's tabs will switch the first pane's content, but not its tab headers.

[polarweasel]

Fixed in Docsy 0.5.1, which we're now using on the live site. No need for this PR.