Validate/highlight when paper-bot lab member slugs are invalid or paper is missing image or missing markdown body

[keller-mark]

Just an idea - the paper-bot script, when opening the PR, could automatically flag if a lab member ID is not present in the member filenames at https://github.com/hms-dbmi/gehlenborglab-website/tree/main/_members (for example, due to a typo). It could also flag when an image is missing, or alt-text for the image is missing, or if the markdown body is empty (missing paper abstract text).

These items could be included in the PR body as a tasklist/checklist for the contributor to review.