📝(documentation) Add custom export templates section

... with usage instructions and limitations

Signed-off-by: rvveber <weber@b1-systems.de>

Author: rvveber

docs/theming.md

@@ -85,4 +85,39 @@ THEME_CUSTOMIZATION_FILE_PATH=<path>
 
 ### Example of JSON
 
-The json must follow some rules: https://github.com/suitenumerique/docs/blob/main/src/helm/env.d/dev/configuration/theme/demo.json
+The json must follow some rules: https://github.com/suitenumerique/docs/blob/main/src/helm/env.d/dev/configuration/theme/demo.json
+
+----
+
+# **Custom Export Templates** 📄
+
+You can define custom export templates to add introductory content, such as headers or titles, to documents before exporting them as PDF or DOCX.
+
+Export Templates are managed through the admin interface and can be selected by users during the export process.
+
+### Benefits
+
+This feature offers several advantages:
+*   **Header customization** 📄: Add custom headers, titles, or branding to exported documents.
+*   **No code changes required** 🔧: Templates are managed through the admin interface without needing developer intervention.
+*   **Flexible content** 🌟: Use HTML to create headers that match your organization's style.
+
+### Limitations ⚠️
+
+- Currently, templates are only prepended to the document.
+More complex layouts are not supported at this time.
+- The `CSS` and `Description` fields are being ignored at this time.
+- <b>Due to technical conversion limitations, not all HTML can be converted to the internal format, see step 4 below!</b>
+
+### How to Use
+1. Make sure a superuser/admin account is available and you have access.
+2. Navigate to the admin interface at `/admin/core/template/add/` and create a new template.
+3. Fill in the `Title` field with a descriptive name for the template.
+4. In the `Code` field, enter the desired HTML content.
+<b>We highly recommend building the header manually in a Doc first, and use the `Copy as HTML` feature to get a compatible HTML structure for this.</b>
+
+5. Check the `Public` checkbox to make the template available in the frontend export modal.
+6. Save the template.
+
+Once created, users can select the template from the export modal in the frontend.
+