refactor: remove asciidoctor-plantuml extension support

[Baris Tanyeri (BarisTanyeri)]

Removes deprecated asciidoctor-plantuml plugin support.

[Copilot]

Pull request overview

This PR removes the deprecated asciidoctor-plantuml plugin/server wiring from the Antora site generation action, leaving Kroki (and Mermaid via Kroki) as the supported diagram-rendering path.

Changes:

• Remove asciidoctor-plantuml from documented supported rendering plugins and from the Docker image global npm installs.
• Stop passing plantuml-server-url into the Antora invocation.
• Remove the PlantUML server container and related dependencies/env vars from the Docker Compose setup and composite action step.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
README.md	Updates documentation to reflect only asciidoctor-kroki as the supported rendering plugin.
entrypoint.sh	Removes the plantuml-server-url attribute from the Antora CLI invocation.
Dockerfile	Drops the global install of asciidoctor-plantuml; keeps Kroki + Lunr extension.
docker-compose.yml	Removes the PlantUML service and its dependency/env wiring; keeps Kroki + Mermaid.
action.yml	Updates action description/step and Compose startup to no longer include PlantUML.

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