Treaing COPYING & its HTML like other trans dirs

If a file in COPYING/ is HTML, it should go in html/ subdir,
and we can now produce a TXT variant based upon it.

docs/Makefile

@@ -14,6 +14,9 @@ LINKS_OPTIONS:=-dump -codepage utf8
 LINKS:=links $(LINKS_OPTIONS)
 
 # FIXME: Support finding files in ??/html/ and ??_??/html/)
+COPYING_HTMLFILES:=$(wildcard COPYING/html/*.html)
+COPYING_TEXTFILES:=$(patsubst COPYING/html/%.html,COPYING/%.txt,$(COPYING_HTMLFILES))
+
 EN_HTMLFILES:=$(wildcard en/html/*.html)
 EN_TEXTFILES:=$(patsubst en/html/%.html,en/%.txt,$(EN_HTMLFILES))
 
@@ -46,7 +49,8 @@ ZH_TW_TEXTFILES:=$(patsubst zh_tw/html/%.html,zh_tw/%.txt,$(ZH_TW_HTMLFILES))
 
 
 .PHONY: all
-all:	$(EN_TEXTFILES) \
+all:	$(COPYING_TEXTFILES) \
+	$(EN_TEXTFILES) \
 	$(ES_TEXTFILES) \
 	$(FR_TEXTFILES) \
 	$(GL_TEXTFILES) \
@@ -60,7 +64,7 @@ all:	$(EN_TEXTFILES) \
 .PHONY: clean
 clean:
 	-rm \
-	$(TEXTFILES) \
+	$(COPYING_TEXTFILES) \
 	$(EN_TEXTFILES) \
 	$(ES_TEXTFILES) \
 	$(FR_TEXTFILES) \
@@ -72,6 +76,9 @@ clean:
 	$(ZH_CN_TEXTFILES) \
 	$(ZH_TW_TEXTFILES)
 
+$(COPYING_TEXTFILES): COPYING/%.txt: COPYING/html/%.html
+	$(LINKS) $< > $@
+
 $(EN_TEXTFILES): en/%.txt: en/html/%.html
 	$(LINKS) $< > $@