Add mono-repo migration notice

[cwillisf]

Proposed Changes

Add a notice at the top of README.md to inform contributors and other developers about our mono-repo and related migration plans.

Reason for Changes

• As opposed to saying nothing until we archive this repository, this notice will hopefully reduce the stress that people who use scratch-gui will experience.
• Folks who can easily move to the new version will benefit from newer features and maintenance earlier.
• Migrating valuable issues and pull requests is a huge task. If contributors are willing to help, letting them know about the change sooner seems better.
• I don't want to cause contributors any more work than necessary; I'd like them to be aware that they can already file issues and PRs against the mono-repo.

Test Coverage

NA, but feedback is welcome.

[Copilot]

Pull Request Overview

This PR adds a migration notice at the top of the README to inform contributors that the scratch-gui repository has been migrated to a mono-repo.

• A prominent visual block quote is added with a link to the new mono-repo and NPM package.
• Minor formatting updates such as modifying URL and note presentations and removing the Troubleshooting section.

Comments suppressed due to low confidence (3)

README.md:93

• The removal of the exclamation mark in the 'Oh no! It didn't work' header results in an inconsistent tone compared to similar headings. Please verify if this change was intentional.

#### Oh no! It didn't work

README.md:116

• [nitpick] The use of underscores for the note formatting differs from the previous usage of asterisks in the file. Please confirm if this stylistic change is intentional.

_NOTE: If you're a Windows user, please run these scripts in Windows `cmd.exe`  instead of Git Bash/MINGW64._

README.md:238

• The removal of the Troubleshooting section related to npm install errors may remove useful guidance for contributors. Please confirm if this removal is intentional.

## Publishing to GitHub Pages