Add pictures to documentation

[betulependule]

I've added a couple of pictures to the documentation (from old presentations, and screenshots of public repos) in places I thought might fit. I would personally remove most of them. Please let me know if there is something that would be better removed or moved elsewhere.

TODO:

• Remove unnecessary pictures (if any)

Fixes #2638

[softwarefactory-project-zuul]

Build failed.
https://softwarefactory-project.io/zuul/t/packit-service/buildset/79aa0ac0df2a4ed7b93be402e871958a

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[softwarefactory-project-zuul]

Build succeeded.
https://softwarefactory-project.io/zuul/t/packit-service/buildset/0f419e1a5e94428e815db6a6966376ba

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[majamassarini]

I love the process-diagram picture! Super helpful.
And also all the others pictures you have added can be really helpful.
Thanks a lot!

[softwarefactory-project-zuul]

Build succeeded.
https://softwarefactory-project.io/zuul/t/packit-service/buildset/3d1922265cf440adb6aa25531068434d

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[softwarefactory-project-zuul]

Build succeeded.
https://softwarefactory-project.io/zuul/t/packit-service/buildset/da386041d56c47ba82dfbd30f376f868

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[softwarefactory-project-zuul]

Build succeeded.
https://softwarefactory-project.io/zuul/t/packit-service/buildset/dbeb6c9cb9ae4c6695eec1cd777f005f

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[betulependule]

Should be fixed now, I hope

Already fixed in a different way

[majamassarini]

LGTM!

[lbarcziova]

looks great!

[softwarefactory-project-zuul]

Build succeeded (gate pipeline).
https://softwarefactory-project.io/zuul/t/packit-service/buildset/b18ed4dd57524f7db0d9e61d8ce57a91

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