20.04 release preparations

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• [x] Check if links are live
• [x] Update guide 20.04 branch
• [x] Test 20.04 installation
• [x] Update screenshots where applicable
• [x] Update guide where necessary
• [x] Remove Warning header
• [x] Merge to master

[kfechter]

Related to #88

[kfechter]

Updating guide partially deferred to the tweaks branch