Add shared textures to documentation

[aussig]

The old website building code which was stripped out when we moved to Wordpress used to list all shared textures for items in the library, with links to the items that share the textures. Would be good to re-implement this.

[aussig]

Partially implemented, but we have an unresolved problem - the info.txt files are built as each item is parsed, so the earlier an item is processed, the fewer shared textures it knows about. We will probably need to move info.txt processing to a second phase, after the initial phase of item processing is complete.