Add format_docs command for documentation trailing space formatting

Makefile

@@ -37,6 +37,9 @@ fmt:
 	@gofmt -s -w .
 	@gci write --custom-order -s standard -s "prefix(github.com/sagernet/)" -s "default" .
 
+fmt_docs:
+	go run ./cmd/internal/format_docs
+
 fmt_install:
 	go install -v mvdan.cc/gofumpt@latest
 	go install -v github.com/daixiang0/gci@latest