Git Migration: Create doc/rest-api when needed.

Create the directory './doc/rest-api' at the start of 'make ari-stubs'
to prevent an error when documentation is generated.  The directory is
also added to git ignores.

ASTERISK-25027
Reported by: Corey Farrell

Change-Id: Iaccc7f0138501c23aa78feaca2f3cce9e68cbc1b

Makefile

@@ -983,6 +983,7 @@ ifeq ($(PYTHON),:)
 	@echo "--------------------------------------------------------------------------"
 	@false
 else
+	@$(INSTALL) -d doc/rest-api
 	$(PYTHON) rest-api-templates/make_ari_stubs.py \
 		rest-api/resources.json .
 endif

doc/.gitignore

 core-en_US.xml
+rest-api
\ No newline at end of file