docs: restore Lumo theme reference

[sissbruecker]

Restores the Lumo theme reference. This only includes some minor changes:

• Moved the reference under components / themes
• Small updates to wording that implied Lumo is the default theme
• Replaced some descriptions with links to existing content in new styling docs (apply light/dark color scheme, loading utility classes)
• Updated source code links to point to CSS files

Did not yet restore cross-references from other articles, let's wait until the content here is reviewed.

[github-actions]

AI Language Review

Color.adoc

1. Use "change the color palette" instead of "switch the color palette" for more precise language in the section on light and dark schemes.
2. The explanation about the ~= operator could be slightly clearer and simplified. Consider revising the sentence: "By using the ~= operator in the [theme~="dark"] selector it'll match the color..." to something like "The ~= operator ensures that the color applies even when multiple theme variants are defined on a single element..."

Elevation.adoc

No issues found.

Style Properties.adoc

No issues found.

Interaction.adoc

No issues found.

Shape.adoc

1. Consistency issue: replace "--lumo-border-radius property is deprecated" with "The --lumo-border-radius property is deprecated."

Size Space.adoc

No issues found.

Typography.adoc

1. Consider replacing "The system font stack is used by default" with "By default, the system font stack is used:" for clarity.

Utility Classes.adoc

1. Correct the capitalization in the title: "Responsive Design" instead of "responsive design."

These comments address clarity and consistency concerns. Ensure that sections are clear and easily comprehensible to a diverse audience.

[rolfsmeds]

Maybe the entire Lumo Variants page is a bit pointless as a separate page. Might as well fold that content into the index page IMO.

Also, let's move the Themes page to the top of the Components subpage list in nav.

[sissbruecker]

Moved the "Light & Dark Color Schemes" and "Compact Preset" sections to the index page and removed the variants page. Also moved "Themes" to the top of the nav.