Build schema documentation with Oxygen

Given there is a valid Oxygen license owned by the company
as described in https://www.oxygenxml.com/oxygen_scripting.html
the schema documentation can be created using Oxygen. This
commit allows building the schema docs with alternative
doc builders and in case of Oxygen would change the way the
schema docs are generated in the following way:

1. Install Oxygen and setup the license to activate the tool

2. Setup the KIWI development shell environment to export
   the schema_tool variable to point to the schemaDocumentation.sh
   script provided by the Oxygen installation

   export oxygen_tool=/path/to/Oxygen/schemaDocumentation.sh

3. In any case of a schema change manually call:

   tox -e schema

   This step was done before as part of the tox doc target
   but can't be done automatically because it would require
   a correctly licensed Oxygen installation in the travis
   environment. Thus the result data has to be part of the
   pull request

4. Build the documentation and review the result

   tox -e doc

5. Create the pull request

.gitignore

@@ -5,6 +5,7 @@ completion/kiwi-ng.sh
 kiwi/schema/kiwi.xsd
 kiwi/schema/xsi.xsd
 kiwi/schema/kiwi.revision
+doc/source/development/schema/xsdDocHtml.css
 
 # C tool binaries
 tools/dcounter

doc/Makefile

@@ -54,7 +54,7 @@ clean:
 
 .PHONY: html
 html:
-	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
+	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html && cp -a source/development/schema/images $(BUILDDIR)/html/development || true
 	@echo
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."