Compress large PNGs (#1057)

Takes a pass at compressing the largest `.png` images across the site.

As Sarunas pointed out in the forums (https://forums.swift.org/t/redesigned-swift-org-is-now-live/80296/13),
the images for the landing page are quite large, where hero-bottom.png
clocks in at 2302kb and swoop-0.png is 1611kb.

Run `pngquant` over the largest images in the site with slightly lossy
compression to improve the first load experience of the site.

`pngquant` was run with the following to compress and replace images in
place:
`pngquant --quality=45-60 -f --ext .png **/*.png`

Some results:
`swoop-0.png`: 1611kb -> 369kb (77% reduction)
`hero-bottom.png`: 2302kb -> 544kb (76% reduction)

PNG retains a broad browser compabitility so it was favoured over using
a new more compressable format like `.webp` or `.avif`

Author: plemarquand