DG-41: Replace master branch with main.

[tormi]

Starting October, all new source code repositories created on GitHub will be named main instead of master as part of the company’s effort to remove unnecessary references to slavery and replace them with more inclusive terms.
Read more: https://github.com/github/renaming#new-repositories-use-main-as-default-branch-name

This PR adds related changes to the WunderFlow workflow and also addresses issue #5.

Todo

Change diagrams, https://github.com/wunderio/WunderFlow/tree/feature/DG-41-master-to-main#diagrams

[tharna]

I added diagram sources under /graphs so they can be easily edited / updated.
These updates should also be applied to gh-pages branch so the public site gets updated as well.

[tormi]

Todo: update diagrams. See also: #14 (comment)

[fede-green]

+1 for this! :)

[misterjoonas]

Who can merge this? @tharna