Write Tutorial about how to use Circles Account in Gnosis Safe App

[adzialocha]

Todo:

• [x] Merge tutorial PR
• [x] Integrate tutorial in navigation sidebar
• [x] Make images look nicer (maybe there is something in docusaurus one can just use)

Related discussion: https://aboutcircles.com/t/use-gnosis-safe-as-circles-wallet/338/11

[louilinn]

Both issue 8 and 9 will be resolved in this PR: https://github.com/CirclesUBI/circles-handbook/pull/23