Issue 001 - Add initial folder structure

Initial folder structure for documentation source files:

- build-files: folder for ant build files for creating output with the
DITA Open Toolkit, such as HTML and PDF
- ditaval-files: folder for ditaval files
- products-core: folder for ditamap files for all documentation products
along with any files that are specific for a product, for example, a
cover image
- products-custom: folder for ditamaps and files for customised
products, for example for a specific library
- topic-library: folders to help organise topic files and images -
initial organised by modules as well as other folders considered
appropriate
- topic-templates: base templates for topics and maps

.gitkeep files used in each folder until content developed.