docs: Create markdown import and rendering guide (#6150)

* docs(start): add guide for rendering markdown content

Add comprehensive guide covering two methods for importing and rendering
markdown in TanStack Start applications:
- Static markdown with content-collections for build-time loading
- Dynamic markdown fetched from GitHub or remote sources at runtime

Includes setup for unified/remark/rehype pipeline, code highlighting with
Shiki, and practical examples for blog posts and documentation pages.

* ci: apply automated fixes

* docs(start): add rendering-markdown to navigation config

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

Author: 3 people

docs/start/config.json

@@ -164,6 +164,10 @@
             {
               "label": "Tailwind CSS Integration",
               "to": "framework/react/guide/tailwind-integration"
+            },
+            {
+              "label": "Rendering Markdown",
+              "to": "framework/react/guide/rendering-markdown"
             }
           ]
         },