Have custom summaries of blog posts

I have no idea about the complexity of this request. I do consider it low priority (feel free to close this if its impractical to implement without to much effort). I've noticed that the blog page on the xmpp.org website shows the first few lines of the blog article. This often doesn't lead to a very good result, with messy texts that contain what appears to be alt-texts of images, or a repeated introduction for similar types of articles (like the monthly newsletter), like in the screenshot below. I believe that things could be improved if a blog article could define an optional, custom summary / introduction, to be used for this purpose (maybe something in the mattermost header). That way, authors can better control what summary is used in listings, which would improve things considerably. Maybe that could even be reused in opengraph type of metadata. <img width="1147" height="1160" alt="Image" src="https://github.com/user-attachments/assets/ee0358f0-0779-44a6-a170-8cd4bc04751f" />