Improve downloads website styling with dark mode and wider layout

[vogella]

Extends sites/eclipse/page.css with a CSS-variable palette (Eclipse-blue accent), automatic dark mode via prefers-color-scheme, Inter and JetBrains Mono typography, card-style tables with larger headers, card-shaped details/accordions and a gradient-stripe popup dialog.

Replaces the section info marker emoji with a FontAwesome fa-solid fa-circle-info icon for a consistent, OS-independent look.

Widens the content container to 1600px and adjusts the main/TOC column split to 80/20 so wide tables have more room.

HTML markup, JSON schema and the build pipeline are untouched.

Fixes #3812

[HannesWell]

Extends sites/eclipse/page.css with a CSS-variable palette (Eclipse-blue accent), automatic dark mode via prefers-color-scheme, Inter and JetBrains Mono typography, card-style tables with larger headers, card-shaped details/accordions and a gradient-stripe popup dialog.

Generally I don't think its good to more than double, almost triple the size of the CSS file. As mentioned in #3812 (comment) one goal is simplicity or at least to make it not too complex.
Would it be possible to leave out few new details to make this significantly simpler?

Inter and JetBrains Mono typography

I have to say that I don't see a noticeable difference between the fonts. But generally it seems to be smaller, especially the titles, which now seem to be even smaller than the following paragraph text.

Replaces the section info marker emoji with a FontAwesome fa-solid fa-circle-info icon for a consistent, OS-independent look.

Please move this into a separate PR. In #3812 (comment) I described the drawbacks of using FontAwesome (of course it has advantages).
But I'm still not sure if this is the overall better solution.
In fact these icons where used in a previous version, which the git history should show. I changed that for the current solution, but that also isn't ideal in all aspects.

Widens the content container to 1600px and adjusts the main/TOC column split to 80/20 so wide tables have more room.

This should also discussed separately. With that applied the TOC looks squashed on long lines like the one for checksums. To solve that a mechanism could be implemented to show a shorter title in the TOC than at the actual section (in that case e.g. just Checksums and Signatures).

[vogella]

Would it be possible to leave out few new details to make this significantly simpler?

Done

Icon and Font changes also removed from the PR.

Widens the content container to 1600px

Also removed from the chnage