Add an index

It's currently a bit awkward to look up the correct usage of a specific function in the spec, because you have to either remember which chapter it's declared in (which is non-obvious for some functions like rput_then_rpc), or do a full text search and sift through false positives.

It would be nice to have a hyperlinked function index at the end of the spec, similar to how the ISO C and C++ specs have. It could optionally also index major concepts, possibly as a separate index.