CLI: Add automigration for GFM in MDX

[ndelangen]

Closes #20091

What I did

I've added an automigration that adds GFM support to addon-docs via adding an addon
I've created the addon that adds GFM config

The addon is a tool to assist users migrate, it's not supposed to be used indefinitely.

The reaosn we did it this way, is because it made writing the auto-migration much much much easier. User will have to do a manual step, but it should get them into a working state

How to test

After running the automigration you should get an extra addon in your addon array
You should also heb the addon added to your package.json
Your storybook should now support GFM without having to configure addon-docs.

There should be a deprecation notice in the command-loine telling you (once again) that you should perform the manual migration.

[socket-security]

Socket Security Pull Request Report

👍 No new dependency issues detected in pull request

Pull request report summary

Issue	Status
Install scripts	✅ 0 issues
Native code	✅ 0 issues
Bin script confusion	✅ 0 issues
Bin script shell injection	✅ 0 issues
Shell access	✅ 0 issues
Uses eval	✅ 0 issues
Unresolved require	✅ 0 issues
Invalid package.json	✅ 0 issues
HTTP dependency	✅ 0 issues
Git dependency	✅ 0 issues
GitHub dependency	✅ 0 issues
New author	✅ 0 issues
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[shilman]

This generally LGTM.

Two points:

1. @yannbf should def review this as he's up on all the latest subtleties around modifying main.js
2. any chance we can add the new addon OUTSIDE of the monorepo? i understand the reasons not to, but the monorepo is just getting bigger & bigger...

[yannbf]

Good stuff @ndelangen ! I made some changes regarding bug fixes and documentation.
Still, this has to be handled: the automigration gets triggered on sb init, which isn't optimal.