Colon Directives / Divs

[rowanc1]

Colon fences are in use in all of the implementations of MyST, however, are not yet defined in the myst-spec.
We should add these and document the current behaviour that we can then improve upon in the future.

The colon_fence extension [..] works exactly the same as normal ``` code fences, but using ::: delimiters.

:::{admonition} The title
:class: note

This note contains *Markdown*
:::

E.g. documented in JupyterBook here:

• https://jupyterbook.org/en/stable/reference/_changelog.html?highlight=colon#deprecations
• https://github.com/executablebooks/MyST-Parser/blob/master/CHANGELOG.md#0130---2020-12-18

I think that this needs to come in before #12, as that may require colon fences.

Some text that I have used on when to use each syntax:

• https://myst-tools.org/docs/mystjs/syntax-overview#directives

Pointers to other projects:

• For fenced blocks, consider requiring exact match on the number of delimiters jgm/djot#132
• Documentation: Clarify fenced_divs jgm/pandoc#4037 (comment)

[chrisjsewell]

As mentioned in #12, I would suggest that perhaps these should no longer be called "colon fences", but instead div (or whatever), and have a somewhat different semantic to fences (i.e. ``` enclosed blocks)