extensions/gen-sample-extension-docs: output Markdown to userpatches/extensions

- otherwise it is lost in a temp dir that is cleaned before the end of run
- same place as the `sample-extension.sh`
- keep in mind original limitations of this still apply:
  - only extension methods _actually used_ by the specific run are included
    - thus, it will _never_ be complete

extensions/gen-sample-extension-docs.sh

@@ -5,7 +5,8 @@ function extension_metadata_ready__499_display_docs_generation_start_info() {
 }
 
 function extension_metadata_ready__docs_markdown() {
-	generate_markdown_docs_to_stdout > "${EXTENSION_MANAGER_TMP_DIR}/hooks.auto.docs.md"
+	mkdir -p "${SRC}/userpatches/extensions"
+	generate_markdown_docs_to_stdout > "${SRC}/userpatches/extensions/hooks.auto.docs.md"
 }
 
 function extension_metadata_ready__docs_sample_extension() {