Use relative image path for downloaded diagrams

[ssaraswati]

Remove the / from _save_kroki_image_and_get_url so that generated mkdocs images can be served on non root domains or within Backstages techdocs.

This more closely matches the previous behavior before HttpMethod option #7

[ghost]

This would break the codebase. The URLs it would provide are only useful for the topmost level of the docs. See my fix in #20 which works for any document.

[b-bittner]

@ssaraswati ,
would you also agree, that #20 would fix your problem also?

[b-bittner]

fixed with #20