Dark and light images for README.md

[cormullion]

This PR adds two images to the README.md of this repo, responding to Github dark and light modes. Github description of the feature here.

They should look like this:

In an earlier PR (#39601) a white line was added, but it looked a bit out of balance to me.

(This is a restrained and simple design to keep the file size down... 😂)

[KristofferC]

It's very big, no?

[cormullion]

It's very big, no?

39 Kb ? - quite small, especially for me. I think the contributor avatars are bigger ...

[ViralBShah]

I thought Kristoffer was referring to it being big visually. It seems fine to me - but in my dark browser, I still see the white version.

[giordano]

but in my dark browser, I still see the white version.

Try to clear the cache, I had the same problem at first.

[cormullion]

to it being big visually

Ah, OK, yes - I just went for social-media-preview size... 🤷‍♂️

I think cacheing may be a thing - I hope github's code works 😉