Let's use pymdownx.snippets for the move

.markdownlintrc

@@ -22,6 +22,9 @@
   // Allow inline HTML
   "MD033": false,
 
+  // First line heading
+  "MD041": false,
+
   // Disable named references validation
   "MD052": false
 }

docs/about/contributing.md

@@ -0,0 +1 @@
+--8<-- "CONTRIBUTING.md"

docs/hooks.py

@@ -4,19 +4,11 @@ import re
 from pathlib import Path
 from typing import TYPE_CHECKING
 
-from mkdocs.structure.files import File, Files
-
 if TYPE_CHECKING:
     from mkdocs.config.defaults import MkDocsConfig
     from mkdocs.structure.nav import Page
 
 
-def on_files(files: Files, config: MkDocsConfig) -> None:
-    f = File('about/contributing.md', config.docs_dir, config.site_dir, config.use_directory_urls)
-    f.abs_src_path = str(Path(config.config_file_path).parent.joinpath('CONTRIBUTING.md'))
-    files.append(f)
-
-
 def _get_language_of_translation_file(path: Path) -> str:
     with path.open(encoding='utf-8') as f:
         translation_line = f.readline()