Remove plantuml dependency

[michaelsauter]

Right now the docs need plantuml.

However, it is only used on one page (contributing / development). I think this does not warrant the extra complexity in configuring and setting it up. The docs are already difficult to contribute to because one needs to know a little about Asciidoctor and Antora, so we should strive to make getting started as simple as possible.

[michaelsauter]

@gerardcl Since you built the diagram - would you be ok with this change?

[gerardcl]

if we can still draw diagrams yes! if not then I guess we will build diagrams aside and then load images?
no prob!

[michaelsauter]

@gerardcl yea can't draw diagrams afterwards anymore. would be annoying, but would make it easier to start documentation locally ...