Add 'MultiSumm' task to 2026 announcement

Added a new task 'MultiSumm' for multimodal summarization of related websites.

Author: cmihaigabriel

_posts/2025-12-17-announcing-2026.md

@@ -8,6 +8,7 @@ We are happy to announce the tasks that will be offered at the next Multimedia E
 * NewsImages: Retrieval and generative AI for news thumbnails
 * Memorability: Predicting the memorability of movie clips and commercial videos
 * Medico: Visual Question Answering (VQA) for gastrointestinal imaging
+* MultiSumm: Multimodal summarization of multiple topically related websites
 
 This year we will offer many of the same tasks as last year to give teams that missed the opportunity in 2025 another chance to participate. Also this year we emphasize insight into the tasks and the data, 
 with a special invitation for “Quest for Insight” papers to be included along with the standard task participation papers in the MediaEval Working Notes Proceedings.