Markdown: allow passing custom executable for markdownlint (#4358)

doc/ale-markdown.txt

@@ -17,6 +17,15 @@ See |ale-dprint-options| and https://dprint.dev/plugins/markdown
 ===============================================================================
 markdownlint                                        *ale-markdown-markdownlint*
 
+g:ale_markdown_markdown_executable     *g:ale_markdown_markdownlint_executable*
+                                       *b:ale_markdown_markdownlint_executable*
+  Type: |String|
+  Default: `'markdownlint'`
+
+  Override the invoked markdownlint binary. You can use other binaries such as
+  markdownlint-cli2.
+
+
 g:ale_markdown_markdownlint_options       *g:ale_markdown_markdownlint_options*
                                           *b:ale_markdown_markdownlint_options*
   Type: |String|