Include the user-manual.html file in the git tree

This way when building Subsurface asciidoc is not necessary at all - except if
you want to work on the user manual and want to see the result of your work.

Previously the build wouldn't fail without asciidoc, but there would be no
working user manual when pressing F1 from within the application. This solves
the problem by shipping the HTML file with the sources.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

Documentation/Makefile

@@ -1,3 +1,12 @@
+#
+# in order for "normal" developers not to have to install asciidoc (which
+# brings in a couple hundred MB of dependencies) we now include both the
+# user-manual.txt and a copy of the generated .html file in git
+#
+# in order to avoid unnecessary thrash with minor version differences, when
+# submitting patches to the user manual, only submit the changes to the .txt
+# file - the maintainer will recreate the .html.git file
+
 DOCNAME = user-manual
 DOCSOURCE = $(DOCNAME).txt
 HTMLDOC = $(DOCNAME).html
@@ -18,8 +27,9 @@ $(OUT)$(DOCNAME).pdf: $(DOCSOURCE)
 	$(A2X) -f pdf $<
 
 $(OUT)$(HTMLDOC): $(DOCSOURCE)
-	@echo "if asciidoc isn't found no HTML documentation is produced but Subsurface is still functional"
-	$(ASCIIDOC) -a toc -a toclevels=3 -a themedir=$(PWD) -a theme=$(THEME) -a stylesdir=$(PWD) $< || true
+	@echo "if asciidoc isn't found the html file included in the sources is copied"
+	$(ASCIIDOC) -a toc -a toclevels=3 -a themedir=$(PWD) -a theme=$(THEME) -a stylesdir=$(PWD) \
+		-o $(HTMLDOC) $< || cp $(HTMLDOC).git $(HTMLDOC)
 
 # Alternatively::
 $(OUT)$(DOCNAME).xhtml: $(DOCSOURCE)