Add directive to show list of abbreviations

[choldgraf]

Users can define abbreviations in their page or project metadata, and these will then be used for each page's content.

Sometimes it's useful to display a list of abbreviations for readers as a reference, similar to a glossary. It'd help if we offered an ```{abbreviations} directive that showed an alphabetical list of all abbreviations across an entire MyST project.

[rowanc1]

Related discussion here:

• https://github.com/orgs/executablebooks/discussions/870
• #1099