fix: rename fetched _configuration_reference.md to .mdx to fix Docusaurus MDX import

Docusaurus MDX requires component-style imports (`<ConfigurationReference />`)
to reference `.mdx` files. The workflow now saves the fetched upstream file
directly as `_configuration_reference.mdx`, and the import in configuration.mdx
is updated to match. The upstream source file is not changed.

Agent-Logs-Url: https://github.com/quickfix-j/quickfixj-pages/sessions/49371692-9f70-4201-be07-80f3b23d2988

Co-authored-by: chrjohn <6644028+chrjohn@users.noreply.github.com>

Author: Copilot

.github/workflows/deploy.yml

@@ -34,7 +34,7 @@ jobs:
         run: |
           curl -sSfL \
             "https://raw.githubusercontent.com/quickfix-j/quickfixj/master/quickfixj-core/src/main/doc/usermanual/usage/configuration.md" \
-            -o docs/_configuration_reference.md
+            -o docs/_configuration_reference.mdx
 
       - name: Setup Node
         uses: actions/setup-node@v6

docs/configuration.mdx

@@ -133,6 +133,6 @@ settings.setString(sessionId, "EndTime", "23:59:00");
 
 ## Full Configuration Options
 
-import ConfigurationReference from './_configuration_reference.md';
+import ConfigurationReference from './_configuration_reference.mdx';
 
 <ConfigurationReference />