doc: Add minimal C and SQL example to add a custom table AM handler

The documentation was rather sparse on this matter and there is no
extension in-core that shows how to do it.  Adding a small example will
hopefully help newcomers.  An advantage of writing things this way is
that the contents are not going to rot because of backend changes.

Author: Phil Eaton
Reviewed-by: Robert Haas, Fabrízio de Royes Mello
Discussion: https://postgr.es/m/CAByiw+r+CS-ojBDP7Dm=9YeOLkZTXVnBmOe_ajK=en8C_zB3_g@mail.gmail.com

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