create shortcode for markdown tabs

[titusfortner]

I'm tempted to put this in draft because it's not the best way to do this, even though it works.

Current docsy theme forces tabs to have highlighting (google/docsy#550)
Would be nice if we could toggle that off on a per-tab basis.

Since I still can't wrap my head around go/hugo enough to do it the right way, I just copied this from the previous docsy implementation when they didn't have automatic highlighting.

I also added padding and a border, but that's about the extent of my design skills.

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[titusfortner]

One of the big advantages of using something like this, imo, is that IntelliJ recognizes Ruby comments (#) as comments when it's inside 3 ticks, but thinks it is an h1 when it is just inside the tab, so it's much harder to work with the document.

[diemol]

This looks nice

Is it possible to remove the black border for the tab?

We can merge this, but also, could you please open a PR to docsy? It'd be nice to have this working upstream and then we just use it without having this custom code.

[titusfortner]

I'm converting this to a draft, because I really want us to find a way to create a toggle for this behavior and make a PR to docsy. I don't like having to specify all the tabs in the tabpane with this PR, it just feels clunky.

I added the border because without the highlight background, it's really hard to see where the tab content ends. We could do an

---

instead. I set it to be an outset border, which makes it more pronounced at the bottom than the top because that's where it is needed.

[titusfortner]

Closed in favor of #915