Add missing reference doc for nth/3 builtin.

[chuckhoupt]

Fixes #196

[chuckhoupt]

You can see built branch at: https://nth-ref-doc--alsprolog.netlify.app/docs/ref/nth.html

[kenbowen]

The nth/3 doc is good to go, despite the following:
It appears that the dfn of nth/3 is currently contained in builtins/simplio.pro, but it was originally in library/listutl2.pro,
as seen by some commented-out exports, along with a companion position/3,4.
If they're going to be in a builtins/*.pro file, both nth/3 and position/3,4 really ought to be in blt_term.pro.
I'll open a low-priority issue to move them. I suspect that neither of us did any "doc processing" on simplio.pro, because the companion position/3,4 are undoc'd like nth/3 was.
I'll open another issue to doc-process simple.pro.

[chuckhoupt]

[re-opened pull after accidental close]

Yea, nth/3 was moved from the libraries to builtins sometime in `96 -- I'm guessing because it was used to define other builtins (i.e. to avoid back-dependencies).