Document how to bring template updates into an existing solution #83

A solution made with dotnet new never gets template updates on its own. Once there's a release to upgrade from, people need to know what changed and what's worth porting.

Phase 1: The page

• guides/upgrade.mdx per template: compare releases, generate a fresh copy with the same options and diff it, port the shared layers first
• Link it from each template's Releases section

Phase 2: Release notes

• Add a short handwritten "If you made a solution before this" section to each release, from the January release on
• Add that step to the releases skill

Checks: the site builds, and the steps work going from the 2026.12.01 release to 2027.01.01.