Allow ServerURL to be set through environment variables

[zhammer]

We use mkdocs-kroki-plugin to render techdocs in our organization's Backstage.

We want to enforce ServerURL while building our docs in the techdocs builder, rather than having users set the ServerURL in their kroki: config.

1. We don't want to risk a user forgetting to set this and sending diagrams to kroki.io
2. We'd like to set different ServerURL per environment

[secustor]

Use KROKI_SERVER_URL that is how we are doing it for Backstage.

[oniboni]

Alternatively you can also use environment variables via mkdocs config:

  - kroki:
      ServerURL: !ENV KROKI_SERVER_URL