docs/sphinx: add compat.py module and nested_parse helper

Create a compat module that handles sphinx cross-version compatibility
issues. For the inaugural function, add a nested_parse_with_titles()
helper that handles differences in line number tracking for nested
directive body parsing.

Spoilers: there are more cross-version hacks to come throughout the
series.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-5-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

docs/sphinx/compat.py

@@ -0,0 +1,35 @@
+"""
+Sphinx cross-version compatibility goop
+"""
+
+from docutils.nodes import Element
+
+from sphinx.util import nodes
+from sphinx.util.docutils import SphinxDirective, switch_source_input
+
+
+def nested_parse_with_titles(
+    directive: SphinxDirective, content_node: Element
+) -> None:
+    """
+    This helper preserves error parsing context across sphinx versions.
+    """
+
+    # necessary so that the child nodes get the right source/line set
+    content_node.document = directive.state.document
+
+    try:
+        # Modern sphinx (6.2.0+) supports proper offsetting for
+        # nested parse error context management
+        nodes.nested_parse_with_titles(
+            directive.state,
+            directive.content,
+            content_node,
+            content_offset=directive.content_offset,
+        )
+    except TypeError:
+        # No content_offset argument. Fall back to SSI method.
+        with switch_source_input(directive.state, directive.content):
+            nodes.nested_parse_with_titles(
+                directive.state, directive.content, content_node
+            )