UG adjustments to existing content, and task-file updates pre-0.13.0

[chalin]

• Contributes to Release 0.13.0 preparation #2266
• Contains revisions of existing sections in the UG for features introduced in 0.13.0
• Updates the Changelog: add a definition of breaking change, so that we're all on the same page, Docsy-project maitainers too. Updates the 0.13.0 entry.
• Adds a Node.js update section to the previously published 0.12.0 blog post
• Adds a 0.13.0 "coming soon" stub.

Preview:

• https://deploy-preview-2369--docsydocs.netlify.app/blog/2025/0.13.0/
• https://deploy-preview-2369--docsydocs.netlify.app/docs/adding-content/navigation/#table-of-contents
• https://deploy-preview-2369--docsydocs.netlify.app/docs/adding-content/shortcodes/#alert