Add blog post about pull from usptream

[lbarcziova]

Fixes #496

I think there are too many screenshots, but I can't decide what to remove :D (maybe the issue in issue_repository doesn't need to be mentioned)... waiting for some feedback on that

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[softwarefactory-project-zuul[bot]]

Build succeeded.

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[softwarefactory-project-zuul[bot]]

Build succeeded.

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[softwarefactory-project-zuul[bot]]

Build succeeded.

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[softwarefactory-project-zuul[bot]]

Build succeeded.

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[lbarcziova]

@majamassarini @jpopelka @FrNecas @lachmanfrantisek thanks for all the suggestions!!

[softwarefactory-project-zuul[bot]]

Build succeeded (gate pipeline).

:heavy_check_mark: pre-commit SUCCESS in 1m 18s