Support Admonition Boxes in Markdown Rendering

The current plan is to support GitHub's syntax. I've already seen it implemented in several packages, including markdown-it and pandoc.

I do plan on extending it to support an optional title. For example,

> [!NOTE]
> Useful information that users should know, even when skimming content.

is their syntax and is implemented with the word Note being displayed. I'll add an optional title such that

> [!NOTE] Keep in mind
> Useful information that users should know, even when skimming content.

would get displayed with Keep in mind rather than Note. Allows for some level of customization and/or internationalization.

I'm also hoping to extend our multiline block quotes to support this. So you could use

>>> [!NOTE]
A large, multiblock note.
>>>

---

Problem

Currently it is not possible to call out specific items when writing issue/merge request/epic descriptions/comments which may be an important concept or an admonition to the user for a common pitfall.

Usecase

When writing documentation it is useful to call out specific items which may be an important concept or an admonition to the user for a common pitfall.

Examples

Sphinx allows this functionality via custom theming of admonitions; see this presentation slide 13 for an example.

Another example from StackOverflow.

Proposal

Support Admonition Boxes in Markdown Rendering

Add a feature to the GitLab markdown that treats admonitions like labels. Users can add their own with the attributes name, header_color, header_text, body_color. They could be used in markdown in the same way as codeblocks, except using >>>NAME instead of ```LANGUAGE.

In the example given as an inline image, the values would be:

field	value
name	Note
header_color	HEX_DARKER_BLUE
header_text	EMPTY
body_color	HEX_LIGHTER_BLUE
The MarkDown would look like:

>>>Note

Let's say the writer wanted the box header to be "Awesome Note" instead of "Note", then the header_text field could be filled in and that would change all occurrences in documentation as well as keep the notation required short.