Add overview of showcase community bundles to docs

[hlxid]

We currently only have the sample bundles per service to showcase how nodecg-io can be used.
To show more complex applications for nodecg-io with more services and to give potential developers some usecase ideas we should add a list of community showcase bundles somewhere in the docs. There people can show off their bundles by creating a pull request to add them to the list and new developers can have a look at them or even use them for their needs.