docs/qapidoc: create qmp-example directive

This is a directive that creates a syntactic sugar for creating
"Example" boxes very similar to the ones already used in the bitmaps.rst
document, please see e.g.
https://www.qemu.org/docs/master/interop/bitmaps.html#creation-block-dirty-bitmap-add

In its simplest form, when a custom title is not needed or wanted, and
the example body is *solely* a QMP example:

```
.. qmp-example::

   {body}
```

is syntactic sugar for:

```
.. admonition:: Example:

   .. code-block:: QMP

      {body}
```

When a custom, plaintext title that describes the example is desired,
this form:

```
.. qmp-example::
   :title: Defrobnification

   {body}
```

Is syntactic sugar for:

```
.. admonition:: Example: Defrobnification

   .. code-block:: QMP

      {body}
```

Lastly, when Examples are multi-step processes that require non-QMP
exposition, have lengthy titles, or otherwise involve prose with rST
markup (lists, cross-references, etc), the most complex form:

```
.. qmp-example::
   :annotated:

   This example shows how to use `foo-command`::

     {body}

   For more information, please see `frobnozz`.
```

Is desugared to:

```
.. admonition:: Example:

   This example shows how to use `foo-command`::

     {body}

   For more information, please see `frobnozz`.
```

Note that :annotated: and :title: options can be combined together, if
desired.

The primary benefit here being documentation source consistently using
the same directive for all forms of examples to ensure consistent visual
styling, and ensuring all relevant prose is visually grouped alongside
the code literal block.

Note that as of this commit, the code-block rST syntax "::" does not
apply QMP highlighting; you would need to use ".. code-block:: QMP". The
very next commit changes this behavior to assume all "::" code blocks
within this directive are QMP blocks.

Signed-off-by: John Snow <jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240717021312.606116-4-jsnow@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

docs/sphinx/qapidoc.py

@@ -27,6 +27,7 @@ https://www.sphinx-doc.org/en/master/development/index.html
 import os
 import re
 import textwrap
+from typing import List
 
 from docutils import nodes
 from docutils.parsers.rst import Directive, directives
@@ -35,6 +36,7 @@ from qapi.error import QAPIError, QAPISemError
 from qapi.gen import QAPISchemaVisitor
 from qapi.schema import QAPISchema
 
+from sphinx.directives.code import CodeBlock
 from sphinx.errors import ExtensionError
 from sphinx.util.docutils import switch_source_input
 from sphinx.util.nodes import nested_parse_with_titles
@@ -538,10 +540,63 @@ class QAPIDocDirective(NestedDirective):
             raise ExtensionError(str(err)) from err
 
 
+class QMPExample(CodeBlock, NestedDirective):
+    """
+    Custom admonition for QMP code examples.
+
+    When the :annotated: option is present, the body of this directive
+    is parsed as normal rST instead. Code blocks must be explicitly
+    written by the user, but this allows for intermingling explanatory
+    paragraphs with arbitrary rST syntax and code blocks for more
+    involved examples.
+
+    When :annotated: is absent, the directive body is treated as a
+    simple standalone QMP code block literal.
+    """
+
+    required_argument = 0
+    optional_arguments = 0
+    has_content = True
+    option_spec = {
+        "annotated": directives.flag,
+        "title": directives.unchanged,
+    }
+
+    def admonition_wrap(self, *content) -> List[nodes.Node]:
+        title = "Example:"
+        if "title" in self.options:
+            title = f"{title} {self.options['title']}"
+
+        admon = nodes.admonition(
+            "",
+            nodes.title("", title),
+            *content,
+            classes=["admonition", "admonition-example"],
+        )
+        return [admon]
+
+    def run_annotated(self) -> List[nodes.Node]:
+        content_node: nodes.Element = nodes.section()
+        self.do_parse(self.content, content_node)
+        return content_node.children
+
+    def run(self) -> List[nodes.Node]:
+        annotated = "annotated" in self.options
+
+        if annotated:
+            content_nodes = self.run_annotated()
+        else:
+            self.arguments = ["QMP"]
+            content_nodes = super().run()
+
+        return self.admonition_wrap(*content_nodes)
+
+
 def setup(app):
     """Register qapi-doc directive with Sphinx"""
     app.add_config_value("qapidoc_srctree", None, "env")
     app.add_directive("qapi-doc", QAPIDocDirective)
+    app.add_directive("qmp-example", QMPExample)
 
     return {
         "version": __version__,