adds an option to only check changed docs (#5028)

Signed-off-by: Wiard van Rij <wiard@outlook.com>

Makefile

@@ -3,6 +3,7 @@ include .busybox-versions
 
 FILES_TO_FMT      ?= $(shell find . -path ./vendor -prune -o -name '*.go' -print)
 MD_FILES_TO_FORMAT = $(shell find docs -name "*.md") $(shell find examples -name "*.md") $(filter-out mixin/runbook.md, $(shell find mixin -name "*.md")) $(shell ls *.md)
+FAST_MD_FILES_TO_FORMAT = $(shell git diff --name-only | grep ".md")
 
 DOCKER_IMAGE_REPO ?= quay.io/thanos/thanos
 DOCKER_IMAGE_TAG  ?= $(subst /,-,$(shell git rev-parse --abbrev-ref HEAD))-$(shell date +%Y-%m-%d)-$(shell git rev-parse --short HEAD)
@@ -218,6 +219,12 @@ docs: build examples $(MDOX)
 	@echo ">> generating docs"
 	PATH="${PATH}:$(GOBIN)" $(MDOX) fmt -l --links.localize.address-regex="https://thanos.io/.*" --links.validate.config-file=$(MDOX_VALIDATE_CONFIG) $(MD_FILES_TO_FORMAT)
 
+.PHONY: changed-docs
+changed-docs: ## Only do the docs check for files that have been changed (git status)
+changed-docs: build examples $(MDOX)
+	@echo ">> generating docs on changed files"
+	PATH="${PATH}:$(GOBIN)" $(MDOX) fmt -l --links.localize.address-regex="https://thanos.io/.*" --links.validate.config-file=$(MDOX_VALIDATE_CONFIG) $(FAST_MD_FILES_TO_FORMAT)
+
 .PHONY: check-docs
 check-docs: ## checks docs against discrepancy with flags, links, white noise.
 check-docs: build examples $(MDOX)