plantuml: add page (#4040)

Co-authored-by: Zlatan Vasović <zlatanvasovic@gmail.com>

pages/common/plantuml.md

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+# plantuml
+
+> Create UML diagrams from a plain text language and render them in different formats.
+> More information: <https://plantuml.com/en/command-line>.
+
+- Render diagrams to default format (PNG):
+
+`plantuml {{diagram1.puml}} {{diagram2.puml}}`
+
+- Render a diagram in given format (e.g. `png`, `pdf`, `svg`, `txt`):
+
+`plantuml -t{{format}} {{diagram.puml}}`
+
+- Render all diagrams of a directory:
+
+`plantuml {{path/to/diagrams}}`
+
+- Render a diagram to the output directory:
+
+`plantuml -o {{path/to/output}} {{diagram.puml}}`
+
+- Render a diagram with the configuration file:
+
+`plantuml -config {{config.cfg}} {{diagram.puml}}`
+
+- Display help:
+
+`plantuml -help`