Support parsed argument to directives

[Mpdreamz]

Various directives in Myst support providing parsed markdown as directive arguments.

```{directive} Argument
```

However I question whether we want to support this at all.

• We want uniformly styled documents
• Typically this is used to bolden text or add colours but this might be a bad practice.

Just for completeness sake, this restriction would not apply to embedded content (Body).

```{directive} Argument
Body
```

[bmorelli25]

Ah. So disabling, for example, the ability to italicize an admonition title:

Makes sense. I'm looking through the MyST docs and struggling to find an example where this adds value.

[KOTungseth]

+1

We only want to support emphasis (ie italics, bold, and monospace) in the body of the text. Never in titles at any level.

As a best practice, we should avoid excessive use of any of these styles to prevent cluttering the text. Emphasis should be reserved for when it truly adds value to the reader’s understanding.