Switch from Prettier to Rumdl for Markdown formatting (#7423)

Rumdl already appears to have _significantly_ fewer bugs than prettier,
and a solid LSP for editor integration.

The tool is: https://github.com/rvben/rumdl/

I've separated out the change across three commits for easier review.

The configuration tries to match the existing formatting, the changes to
the all the files are to correct issues found by the new tool.

Assisted-by: Antigravity with Gemini

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

testing/file_test/README.md

@@ -84,6 +84,9 @@ they have an associated error. An exception is that the main test file may omit
 
 ## Content replacement
 
+<!-- TODO: Re-enable once rumdl's bugs with complex nested lists are fixed. -->
+<!-- rumdl-disable -->
+
 Some keywords can be inserted for content:
 
 -   ```
@@ -289,6 +292,8 @@ Supported comment markers are:
     run the test directly. Tips have no impact on validation; the marker informs
     autoupdate that it can update or remove them as needed.
 
+<!-- rumdl-enable -->
+
 <!--
 {% endraw %}
 -->