docs: new styling docs / utility classes

[sissbruecker]

Adds the "Utility Classes" article for the new styling docs.

Note

The PR targets a base branch for the new styling docs. That branch has the old styling docs removed and new articles are added gradually until the new section is ready. Cross-references between new articles will be filled in later, respective sections have been marked with TODOs.

[github-actions]

AI Language Review

• The sections marked with // TODO: should be completed with the appropriate links to enhance the document's usefulness.
• The phrase "Loading Lumo Utility Classes through theme.json is no longer supported" should be adjusted for consistency and clarity. Consider rephrasing or adding context to help readers understand this change better.