Collection of pointers to directives without content

[rowanc1]

There is some ability in other markup languages to supply directives without content, MyST right now is a bit awkward and confuses some folks:

• https://github.com/orgs/executablebooks/discussions/824
• https://github.com/executablebooks/MyST-Parser/issues/617

Other implementations:

• https://www.npmjs.com/package/remark-directive (leafDirective)

This issue is just for collecting pointers to various places where we might want to think about improvements to the spec in the future!

[rowanc1]

Another reference point is directives in sphinx, which can be on a single line: