Add GitLab DoS announcement

[wwarriner]

Pull Request

Intent

Add announcement describing GitLab DoS.

Proposed Changes

• Adds announcement.
• Fixes announcement rendering mechanism in main index.md file.

Changes to Section Headers

Changes to Page URLs

Related or Fixed Issues

Accessibility Checklist

I have ensured all of the following for new or changed content:

• all images have appropriate alt text. Guidelines
• all images with text have sufficient contrast ratio. Checker
• all image text is transcribed or described in body text.
• all technical terms, jargon, and abbreviations are introduced before they are used.

Style Checklist

I have done all of the following:

• searched for other relevant pages and crosslinked from my content to them.
• searched for other relevant pages and crosslinked from them to my content.
• added redirects in mkdocs.yml for any moved headers or pages.
• defined new technical terms, jargon, and abbreviations in the glossary.
• searched for existing technical terms, jargon, and abbreviations in the glossary and added crosslinks to them.
• properly formatted and introduced key branding terms. List here.