Add --skip-solid flag to rails new command in README

[Copilot]

Why?

The documented rails new command in the README was missing the --skip-solid flag, which excludes Solid Queue/Cache/Cable defaults when scaffolding a new app.

What Changed

• Append --skip-solid to the rails new command in README.md:

rails new <app-name> --javascript=webpack --database=postgresql --skip-test --skip-solid

Pre-merge checklist

• Update relevant READMEs
• Run bin/bump_version or bin/bump_version --patch

Screenshots

N/A — documentation only.