docs(preview): add imaginary pdf provider

Signed-off-by: Daniel Kesselberg <[email protected]>

Author: kesselb

admin_manual/release_notes/upgrade_to_30.rst

@@ -24,3 +24,9 @@ Changes to the available options in ``config.php``.
 * The option ``forbidden_chars`` is now deprecated and replaced with ``forbidden_filename_characters``
 * The option ``forbidden_filename_basenames`` was added to allow bocking files with specific basenames (the filename without extension (before the first dot))
 * The option ``forbidden_filename_extensions`` was added to allow blocking extensions from being used on filenames
+
+Previews for PDF files with Imaginary
+-------------------------------------
+
+The preview provider ``OC\Preview\Imaginary`` is no longer generating previews for PDF files.
+Add the new preview provider ``OC\Preview\ImaginaryPDF`` to ``enabledPreviewProviders`` to enable preview generation with Imaginary for PDF files.