Modularize generated-documentation creation code

Make modules out of lily-sort.scm and all document-*.scm files. This
fixes an eleven-year old TODO comment from documentation-generate.scm.

In the process, reorganize the file structure of Internals Reference
generation code to better fit the new infrastructure, and add more
source comments.

Also, clean up the oop interfaces. Create new base classes
<internal-item-doc> for IR bottom-level documentation nodes and
<internal-type-doc> for immediate supernodes of these; this allows to
avoid some code duplication e.g. between <context-type-doc> and
<translator-type-doc>. Consolidate on a slot name 'low-level-record'
for encapsulated documentation data in all documention item
subclasses.