[i18n] Add a `make pot` command to Makefile (#3006)

By analogy with <https://github.com/FreshRSS/FreshRSS/pull/2996>.

Makefile

@@ -150,6 +150,10 @@ endif
 rtl: ## Generate RTL CSS files
 	rtlcss -d p/themes && find . -type f -name '*.rtl.rtl.css' -delete
 
+.PHONY: pot
+pot: ## Generate POT templates for docs
+	cd docs && ../cli/translation-update.sh
+
 ##########
 ## HELP ##
 ##########