doc: git mv to .md

* doc: rename .markdown references in content
* doc: rename to .md in tools
* doc: rename to .md in CONTRIBUTING.md

PR-URL: #4747
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: techjeffharris
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

Author: eljefedelrodeodeljefe

CONTRIBUTING.md

@@ -131,7 +131,7 @@ test/gc/node_modules/weak/build/Release/weakref.node: $(NODE_EXE)
 		--nodedir="$(shell pwd)"
 
 # Implicitly depends on $(NODE_EXE), see the build-addons rule for rationale.
-test/addons/.docbuildstamp: doc/api/addons.markdown
+test/addons/.docbuildstamp: doc/api/addons.md
 	$(RM) -r test/addons/??_*/
 	$(NODE) tools/doc/addon-verify.js
 	touch $@
@@ -239,9 +239,9 @@ test-v8-benchmarks:
 test-v8-all: test-v8 test-v8-intl test-v8-benchmarks
 	# runs all v8 tests
 
-apidoc_sources = $(wildcard doc/api/*.markdown)
-apidocs = $(addprefix out/,$(apidoc_sources:.markdown=.html)) \
-		$(addprefix out/,$(apidoc_sources:.markdown=.json))
+apidoc_sources = $(wildcard doc/api/*.md)
+apidocs = $(addprefix out/,$(apidoc_sources:.md=.html)) \
+		$(addprefix out/,$(apidoc_sources:.md=.json))
 
 apidoc_dirs = out/doc out/doc/api/ out/doc/api/assets
 
@@ -258,10 +258,10 @@ out/doc/api/assets/%: doc/api_assets/% out/doc/api/assets/
 out/doc/%: doc/%
 	cp -r $< $@
 
-out/doc/api/%.json: doc/api/%.markdown $(NODE_EXE)
+out/doc/api/%.json: doc/api/%.md $(NODE_EXE)
 	$(NODE) tools/doc/generate.js --format=json $< > $@
 
-out/doc/api/%.html: doc/api/%.markdown $(NODE_EXE)
+out/doc/api/%.html: doc/api/%.md $(NODE_EXE)
 	$(NODE) tools/doc/generate.js --format=html --template=doc/template.html $< > $@
 
 docopen: out/doc/api/all.html