docs(diagrams): add Makefile to render PlantUML and Graphviz PE-4997

We'd like to use PlantUML and in particular C4 to illustrate gateway
architecture so that it's easier for new developers to get up to speed
on it. This adds a Makefile that downloads the PlantUML jar and uses it
to render PlantUML diagrams as PNGs. Also, since I copied this from
another project that had the functionality already and it's generally
useful, it renders Graphviz dot diagrams too.

.gitattributes

@@ -1,4 +1,5 @@
 /docker-entrypoint.sh text eol=lf
 /envoy/docker-entrypoint.sh text eol=lf
 /envoy/install-ytt.sh text eol=lf
-**/*.sh text eol=lf
+**/*.sh text eol=lf
+*.png filter=lfs diff=lfs merge=lfs -text

Makefile

@@ -0,0 +1,25 @@
+# Renders PlantUML and Graphviz dot diagrams source files in 'docs/diagrams'
+
+.PHONY: all dot_files puml_files
+
+plantuml_version := 1.2023.12
+plantuml_jar := plantuml-$(plantuml_version).jar
+plantuml_jar_path := vendor/$(plantuml_jar)
+
+dot_pngs := $(patsubst docs/diagrams/%.dot,docs/diagrams/%.png,$(wildcard docs/diagrams/*.dot))
+puml_pngs := $(patsubst docs/diagrams/%.puml,docs/diagrams/%.png,$(wildcard docs/diagrams/*.puml))
+
+all: $(plantuml_jar_path) $(dot_pngs) $(puml_pngs)
+
+# Download PlantUML jar
+$(plantuml_jar_path):
+	mkdir -p vendor
+	curl -L -o $(plantuml_jar_path) https://github.com/plantuml/plantuml/releases/download/v$(plantuml_version)/$(plantuml_jar)
+
+# Render Graphviz dot files
+$(dot_pngs): docs/diagrams/%.png: docs/diagrams/%.dot
+	dot -Tpng -o $@ $<
+
+# Render PlantUML files
+$(puml_pngs): docs/diagrams/%.png: docs/diagrams/%.puml
+	java -jar $(plantuml_jar_path) -o $(CURDIR)/docs/diagrams -tpng $<