Define layout engine in d2 file (#9697)

## What changed?

Moved decision which d2 layout engine to use into file.

## Why?

Allows each file to pick the right layout engine for itself.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/Makefile

@@ -4,8 +4,8 @@ DIAGRAMS := $(wildcard ./_assets/*.d2)
 
 install-d2:
 	@printf $(COLOR) "Install d2..."
-	@go install oss.terrastruct.com/d2@v0.6.3
+	@go install oss.terrastruct.com/d2@v0.7.1
 
 update-diagrams: install-d2
 	@printf $(COLOR) "Update diagrams"
-	$(foreach DIAGRAM, $(DIAGRAMS), $(shell d2 --layout elk --theme 7 --dark-theme 200 $(DIAGRAM) $(NEWLINE)))
+	$(foreach DIAGRAM, $(DIAGRAMS), $(shell d2 --theme 7 --dark-theme 200 $(DIAGRAM) $(NEWLINE)))

docs/_assets/matching-context.d2

@@ -1,6 +1,12 @@
 # https://d2lang.com
 # Generate SVG by running `make` inside of `docs/`.
 
+vars: {
+  d2-config: {
+    layout-engine: elk
+  }
+}
+
 Cluster: {
     grid-rows: 2
     vertical-gap: 10

docs/_assets/retries.d2

@@ -1,6 +1,12 @@
 # https://d2lang.com
 # Generate SVG by running `make` inside of `docs/`.
 
+vars: {
+  d2-config: {
+    layout-engine: elk
+  }
+}
+
 classes: {
     invisible: {
         style.opacity: 0