build(docs): output pages as pagename/index.html for clean URLs

Split DOCS_OUT into INDEX_OUT (index.md files stay as index.html) and
PAGE_OUT (all other pages become pagename/index.html). The __ROOT__
depth computation already handles the extra directory level correctly.

Makefile

@@ -11,8 +11,12 @@ SIDECAR_EMBED := internal/sidecar/assets/sidecar-all.jar
 SDK_AAR := sdk/android/build/outputs/aar/sdk-android-release.aar
 SANDERLING_BIN := bin/sanderling
 
-DOCS_SRC := $(shell find docs -type f -name '*.md' -not -path 'docs/_*')
-DOCS_OUT := $(patsubst docs/%.md,build/site/%.html,$(DOCS_SRC))
+DOCS_SRC      := $(shell find docs -type f -name '*.md' -not -path 'docs/_*')
+INDEX_SRC     := $(filter %index.md,$(DOCS_SRC))
+PAGE_SRC      := $(filter-out %index.md,$(DOCS_SRC))
+INDEX_OUT     := $(patsubst docs/%.md,build/site/%.html,$(INDEX_SRC))
+PAGE_OUT      := $(patsubst docs/%.md,build/site/%/index.html,$(PAGE_SRC))
+DOCS_OUT      := $(INDEX_OUT) $(PAGE_OUT)
 DOCS_TEMPLATE := docs/_template/page.html
 
 INSPECT_DIST := internal/inspect/dist
@@ -90,12 +94,19 @@ build/site/_assets: docs/_assets
 	@rm -rf $@
 	@cp -R $< $@
 
-build/site/%.html: docs/%.md $(DOCS_TEMPLATE)
+define build_page
 	@mkdir -p $(dir $@)
 	@pandoc $< --from=gfm --to=html5 --standalone \
 	  --highlight-style=tango --template=$(DOCS_TEMPLATE) -o $@
 	@rel=$$(echo $(patsubst build/site/%,%,$@) | awk -F/ '{for(i=1;i<NF;i++)printf "../"}'); \
 	  sed -i.bak "s|__ROOT__|$$rel|g" $@ && rm $@.bak
+endef
+
+$(INDEX_OUT): build/site/%.html: docs/%.md $(DOCS_TEMPLATE)
+	$(build_page)
+
+$(PAGE_OUT): build/site/%/index.html: docs/%.md $(DOCS_TEMPLATE)
+	$(build_page)
 
 clean:
 	$(GO) clean