Atlas Infra handbook launch

The Atlas Infra handbook is now structured as a Docusaurus site with:

• a dedicated homepage
• a permanent docs area for runbooks and reference
• a blog area for future operational updates and changelog-style entries

This blog is intentionally narrow in scope. It should be used for:

• infrastructure updates worth announcing
• operational changes that are time-bound
• release-style notes about the handbook itself

Permanent architecture and reference material still belongs in /docs, not in the blog.