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

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e09fff7c980a8b545a18e119be41bf67cd2e9182

Modified Files
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doc/src/sgml/tableam.sgml | 51 +++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 49 insertions(+), 2 deletions(-)