👌 offset nested headers by current heading level

This makes it so nested headings (in blockquotes, admonitions, etc.)
generates rubrics that are offset by the current heading level,
meaning that:

```markdown
# Title
## Sub-title
> Quote:
> # Title in quote
> Content.
```

will generate a rubric level 3 (level 1 + offset by 2 by the "Sub-title")
inside the blockquote.

This also makes those rubrics support the Sphinx option `heading-level`,
for properly generating `<h1>`, `<h2>`, etc.

Author: minijackson

myst_parser/mdit_to_docutils/base.py

@@ -825,8 +825,11 @@ def render_heading(self, token: SyntaxTreeNode) -> None:
             parent_of_temp_root
             or isinstance(self.current_node, nodes.document | nodes.section)
         ):
+            level_offset = max(self._level_to_section.keys())
+            level += level_offset
             # if this is not the case, we create a rubric node instead
             rubric = nodes.rubric(token.content, "", level=level)
+            rubric["heading-level"] = level
             self.add_line_and_source_path(rubric, token)
             self.copy_attributes(token, rubric, ("class", "id"))
             with self.current_node_context(rubric, append=True):

tests/test_renderers/fixtures/docutil_syntax_elements.md

@@ -113,12 +113,19 @@ Heading Levels:
 
 Nested heading
 .
+# Main heading
 > # heading
+> ## sub-heading
 .
 <document source="notset">
-    <block_quote>
-        <rubric ids="heading" level="1" names="heading">
-            heading
+    <section ids="main-heading" names="main\ heading">
+        <title>
+            Main heading
+        <block_quote>
+            <rubric heading-level="2" ids="heading" level="2" names="heading">
+                heading
+            <rubric heading-level="3" ids="sub-heading" level="3" names="sub-heading">
+                sub-heading
 .
 
 Block Code:

tests/test_renderers/fixtures/sphinx_syntax_elements.md

@@ -113,12 +113,19 @@ Heading Levels:
 
 Nested heading
 .
+# Main heading
 > # heading
+> ## sub-heading
 .
 <document source="<src>/index.md">
-    <block_quote>
-        <rubric ids="heading" level="1" names="heading">
-            heading
+    <section ids="main-heading" names="main\ heading">
+        <title>
+            Main heading
+        <block_quote>
+            <rubric heading-level="2" ids="heading" level="2" names="heading">
+                heading
+            <rubric heading-level="3" ids="sub-heading" level="3" names="sub-heading">
+                sub-heading
 .
 
 Nested heading in object