Enable the 'admonitions' Markdown extension for Mkdocs.

The admonitions extension to Python-Markdown allows you to include
rST-style "admonitions" to Markdown documents, for instance,

    !!! note
        Here's an important note to keep in mind!

In general, the current documentation uses bold text to try to achieve
the same effect, which is a bit harder to notice and makes it difficult
to differentiate between "here's something useful to know" versus "here
there be dragons".

mkdocs.yml

@@ -5,6 +5,9 @@ copyright: GoToSocial is licensed under the GNU AGPL v3 LICENSE. Copyright (C) 2
 plugins:
   - render_swagger
 
+markdown_extensions:
+  - markdown.extensions.admonition
+
 extra_javascript:
   - assets/js/swagger-ui-bundle.js