Compress images

[mueller-ma]

Images could be compressed to improve loading times of the website. There's a handy GitHub bot for that: https://github.com/marketplace/imgbot
I added it to my personal fork of this repo: https://github.com/mueller-ma/website/pull/1

[ghys]

We can give it a try, but it would only cover the images that are on this repo (the bulk of them are archived docs of previous versions), not those which are imported from other repos as part of the build process.

[ghys]

I run imgbot in the docs repo to try to optimize the logos (openhab/openhab-docs#1222, for the next.openhab.org website, not the current) - since they appear on the home page, this marginally helps reducing its weight.