build(aio): map H3 headings into H4 headings for certain templates (#24000)

The sections such as methods and decorator options are already headed
by a H3 heading so we need to map the H3 headings in the API doc source
down to H4 headings.

This commit includes general heading mapping functionality accessible via
the `marked` Nunjucks filter.

PR Close #24000

aio/tools/transforms/remark-package/services/renderMarkdown.spec.js

@@ -95,4 +95,36 @@ describe('remark: renderMarkdown service', () => {
     '</code-example>\n'
     );
   });
+
+  it('should map heading levels as specified', () => {
+    const content =
+    '# heading 1\n' +
+    '\n' +
+    'some paragraph\n' +
+    '\n' +
+    '## heading 2a\n' +
+    '\n' +
+    'some paragraph\n' +
+    '\n' +
+    '### heading 3\n' +
+    '\n' +
+    'some paragraph\n' +
+    '\n' +
+    '## heading 2b\n' +
+    '\n' +
+    'some paragraph\n' +
+    '\n';
+    const headingMappings = { h2: 'h3', h3: 'h5' };
+    const output = renderMarkdown(content, headingMappings);
+    expect(output).toEqual(
+      '<h1>heading 1</h1>\n' +
+      '<p>some paragraph</p>\n' +
+      '<h3>heading 2a</h3>\n' +
+      '<p>some paragraph</p>\n' +
+      '<h5>heading 3</h5>\n' +
+      '<p>some paragraph</p>\n' +
+      '<h3>heading 2b</h3>\n' +
+      '<p>some paragraph</p>\n'
+      );
+  });
 });