Add support for Markdown-style triple backticks for code/listings/examples/verbatim/...

[fingolfin]

It would be nice if one could use backticks as in (github flavored) MarkDown as a shortcut for inserting code. We already have single backticks (as alias for <C>foo</C>), but it would be nice to also have triple backticks, as alias for

<Listing><![CDATA[
...
]]></Listing>

Of course the devil is in the details, parsing those leave all kinds of corner cases... Still, would be highly useful, IMHO.

[fingolfin]

Actually, one might want to do a bit more to support all of <Listing>, <Example>, <Log>: I'd imitate what GH flavored Markdown does to indicate languages:

```listing
# this is a <Listing>
f := x->1;
```

```example
# this is an <Example>
gap> 1+1;
2
```

```log
# this is a <Log>
gap> 1+1;
2
```

Based on how frequently I use them, I think I'd actually set the default to "example".

[sebasguts]

This should actually be not too hard to do, with the problem that it might break a lot.

With not too much work, I would guess it could be added here: https://github.com/gap-packages/AutoDoc/blob/master/gap/Markdown.gi#L144

[fingolfin]

@sebasguts why do you think this would break a lot? Are you aware of any packages using triple backquotes in their AutoDoc code (which of course right now would be transferred as-is into the GAPDoc XML)?

[fingolfin]

Ah, perhaps you were talking about single backticks? Sure, those are more likely to be used somewhere, and thus cause problems. Well, I just run

rg --type=gap  '#!.*`'

on the GAP package distribution as well as all packages repositories I know about, and only found a single hit inside curlInterface (trivially to be fixed), and two in my abandoned UnitTests package. So I think we are good on that front. I also checked all *.autodoc files I could find: nothing there.