Draft: Style tweaks, first pass: Index Extravaganza

I had a few gripes with the new page design; this MR acts as an initial pass at style tweaks to try to make it look a bit more complete. (I was planning to participate in the initial design process, but was busy with many other things, and forgot about it until the site went live - sorry!)

A general rundown:

• All fonts are replaced with Open Sans. It's the font we use for the postmarketOS logo during boot splash generation, and it happens to also be a really nice font for both headers and body text. This lets us shave about a megabyte off the content that needs to be downloaded, and matches the website a lot better instead of having mismatched fonts everywhere.
• Colors are changed for both light and dark modes - the new colors have a green-ish tint matching the accent color. For some reason, the old dark theme colors were actually blue-ish - this is now fixed to be more in-line with the general design.
• The index header ("start banner") now has a raised background, and the Explore button from the About section is moved to it. This is somewhat inspired by e.g. Flathub's app banners.
• Headers are now bold and smaller.
• The index page is reorganized and has various tweaks: the "User interfaces" section is moved below About and no longer has a background, the image next to the About section is re-added to make the page feel a bit less empty, the quotes have been redesigned...
• Most of the changes are for the index page, but I threw in a bonus for blog posts as well - the title and date/reading time get some more spacing. I'll focus on blog post improvements separately.

Of course, this is all my subjective opinion - feedback is highly appreciated :) (and thanks a lot to the redesign crew, I might sound a bit negative in this MR but I do appreciate your work!)

I tried to split everything up into separate commits as best as I could, but my rapid iteration meant that some stuff got joined together. If I should rebase and split these commits up, let me know.

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