Updated code style guide

[przemyslaw-zan]

🚀 Summary

Added section dedicated to custom stylelint rules in the code style guide.

Bumped the stylelint-config-ckeditor5 to v12

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📌 Related issues

• Closes Stylelint: Names of CSS variables within .ck-content should follow the --ck-content-* pattern #18805

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💡 Additional information

Requires following changes to be released:

• Added variable name rule ckeditor5-linters-config#72

[CKEditorBot]

📢 Changelog entry reminder

Hi there! 👋

Thanks for opening this pull request! If your changes affect end users, please add a changelog entry to describe them.
We use a Markdown-based system - see the Changelog entries guide for details.

✍️ How to

Run the following command:

yarn run nice

This guide will help you create a new changelog file. You can add as many entries as needed – one per distinct change.

Note

Skip internal changes, such as typo fixes or refactors, as they won't be included in the published changelog.

Tip

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[pomek]

LGTM.