Ship a starter README in every generated solution #79

A generated solution only gets docs/RUN.md, so when someone pushes it to GitHub the front page is empty. A short starter README fixes that and points to the docs and AGENTS.md.

Phase 1: The README

• Add template/README.md to each template: the solution name (renamed by sourceName), what it is, how to run and test it, links to RUN.md, AGENTS.md and the docs site
• Pack it as content/README.md in template/template.csproj and add it to the .slnx solution items
• Keep the template's own name out of every URL in it
• Update template-authoring and the drift list if the file is shared

Phase 2: Docs

• Add it to each reference/project-structure.mdx

Checks: the build pipeline passes, Verify Template shows README.md in the generated solution, and Shared Layers Drift finds no differences.