Announce R-U GSoD, stop using preamble for R-U posts (#752, #754))

[steffilazerte]

• post follows Content Guidelines
• post follows Style Guide
• title is in Title Case
• publication date is ok
• author metadata is provided with correct folder name
• html not included in pull request of Rmd post
• I ran roblog::ro_lint_md() on index.md
• I ran roblog::ro_check_urls() on index.md
• I ran a spell-check on index.md
• YAML subject tags are ok ("tech notes" for tech notes; "community" for non-staff non-editor)
• YAML field 'preface' is present if necessary

[steffilazerte]

@maelle I think we need to rethink how the preamble automatically shows up on anything tagged with R-Universe... It also may not be necessarily for all these posts, now that the documentation will have a proper home 😁

[maelle]

I opened #754.

[noamross]

I've combined this and #754, which now drops the preamble only for new posts after 2024-04-10. Preview confirms that it works, I will go ahead and merge. @steffilazerte, you can post announcements. Thanks!