Copy specifications from module specs directory to docs image

JEP 299 outlines a plan to store specifications in module-specific specs directories, such that they can be copied to the specs directory in the docs image.

We need to update the rules to build the docs image to either copy these files to the specs directory, or to run the files through tools as needed. For example, Markdown files should be run through pandoc. We might optionally consider running html files through tidy. Additional supporting files, such as images should be transferred as well.