Add imgbot to auto-optimize images

[coliff]

Your image file sizes would be reduced by 31% 🎉

Details

File	Before	After	Percent reduction
/static/images/favicon.png	1.22kb	0.59kb	51.48%
/exampleSite/content/basics/installation/images/chapter.png	85.71kb	46.66kb	45.56%
/exampleSite/content/cont/pages/images/pages-chapter.png	243.33kb	152.42kb	37.36%
/exampleSite/content/basics/style-customization/images/red-variant.png	380.82kb	239.39kb	37.14%
/exampleSite/content/basics/style-customization/images/blue-variant.png	380.01kb	238.93kb	37.13%
/exampleSite/content/basics/style-customization/images/green-variant.png	381.74kb	240.20kb	37.08%
/exampleSite/content/cont/pages/images/pages-default.png	310.79kb	196.47kb	36.78%
/exampleSite/static/images/showcase/tat.png	405.21kb	302.69kb	25.30%
/static/images/gopher-404.jpg	215.36kb	201.42kb	6.47%
/images/tn.png	141.37kb	139.98kb	0.99%
Total :	2,545.56kb	1,758.74kb	30.91%

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[matcornic]

Thanks for the tip, I added the service.

I tried to force request the optimisation on Imgbot website, but I did not see any PR for now.

Let's give the bot some time :)

[coliff]

cool - I see imgbot worked his magic in #222. :-)