Add sidebar for desktop layout on landing

[meodai]

Introduces the possibility to add a sidebar on desktop.

mainWindow now comes with two more slots slotSidebar and slotSidebarDecoration.

• slotSidebar holds the main content of the sidebar
• slotSidebarDecoration is used for the a visual of the dapps explorer

For now those only show marketing content, so they are completely hidden on mobile. This might change in the future if needed.

Notable CSS changes:

• Add CSS for the sidebar itself: https://github.com/dfinity/internet-identity/pull/1705/files#diff-dfcfd73ef8243285e586ac8980bd197e24b434855418783109c0404283304b23R628-R664
• Put "design gutter" into a variable for easier reuse: https://github.com/dfinity/internet-identity/pull/1705/files#diff-dfcfd73ef8243285e586ac8980bd197e24b434855418783109c0404283304b23R138
• Add new reference colors and sizes for the sidebar https://github.com/dfinity/internet-identity/pull/1705/files#diff-dfcfd73ef8243285e586ac8980bd197e24b434855418783109c0404283304b23R171-R249
• Replace min-width and max-width both set to 40rem by just width: https://github.com/dfinity/internet-identity/pull/1705/files#diff-dfcfd73ef8243285e586ac8980bd197e24b434855418783109c0404283304b23L625-R668
• Make sure the .l-wrap styles are not applied to #pageContent as well

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🟢 Some screens were added

🟡 Some screens were changed

🔴 Some screens were removed

[nmattia]

I think there's some padding/spacing that went missing here!

[nmattia]

It looks like the whole page got narrower here, is that expected?

[meodai]

good catch! Il investigate!

[nmattia]

I think there's a footer issue here; plus shouldn't there be a sidebar?

[meodai]

I think there is an issue with position fixed on the screenshot thingy, at least it works in real life.

[meodai]

replaced by #1732