Improve downloads page

[mooskagh]

Stockfish recently updated their downloads page, and it looks very convenient.

It would be nice if we made something similar.
In our case it's more challenging as I'd say we also need (similarly well structured) networks page.

[john-sp]

I will claim this. I believe the approach I'm taking is simpler for users compared the old version without a direct copy of Stockfish's design. I'd appreciate any feedback before submitting the PR. Also, note that the GPU panels are initially closed

[mooskagh]

Sounds good!

See also #134 where I just asked some LLM to improve the "Quickstart" page, and then it turned out to be very similar to the download page, so I removed download page but wasn't sure whether I like it.

I believe we do need the download page, so improving it sounds good.