Provide an index / summary interface for easy access to LFE and library docs

[oubiwann]

Part of epic #28

[oubiwann]

If we do all docs in HTML format with lodox (as pondered in this comment: #29 (comment)), we can simply create an index file that links to LFE's lodox pages and each library's lodox pages, and we're done :-) (well, done as far as creating a common interface for linking docs is concerned, anyway ...)