Add original resolution background image

Add original resolution background image

- A photo of a rainbow flag is used as the website background and `README.md` header image.
- The current image is $(1024*681)/(3008*2000) \approx 0.12 \approx 1/8$ the size (in pixels) of the original image.
- Due to the smaller scale, and perhaps high compression by Wikipedia, "pixelization" and image compression artifacts are visible on some (wide) screens.
- Retrieved the original 3008x2000 pixel photo by Ludovic Bertron from Wikipedia (CC-BY 2.0).
- Compressed for web using `imagemagick`; from 2.9 MB to 485 kB. There are some artifacts, but not as clearly visible due to the higher resolution.
- Keeping the 1024x681 pixel (132 kB) image, for now, since it has been used for long time and might "break" older revisions of `README.md` etcetera.

Resize command used:

```shell
convert Rainbow_flag_and_blue_skies.jpg \
    -sampling-factor 4:2:0 \
    -strip \
    -quality 85 \
    -interlace Plane \
    -gaussian-blur 0.05 \
    -colorspace RGB \
    3008px-Rainbow_flag_and_blue_skies.jpg
```

See

- https://en.wikipedia.org/wiki/File:Rainbow_flag_and_blue_skies.jpg
- https://dev.to/feldroy/til-strategies-for-compressing-jpg-files-with-imagemagick-5fn9
- https://stackoverflow.com/questions/7261855/recommendation-for-compressing-jpg-files-with-imagemagick