Not clear that posters are non-archival

[dylancashman]

We got lots of questions (from last year) about posters being archival: can we make a bit more prominent on the webpage the phrasing “Posters are non-archival….”?

@wjwillett can you or one of the poster chairs make a pull request with updated language? Potential pages could be https://github.com/ieee-vgtc/ieeevis.org/blob/vis2024/content/info/call-participation/posters.md or https://github.com/ieee-vgtc/ieeevis.org/blob/vis2024/content/info/call-participation/posters-faq.md