Add minimal C example and SQL registration example for custom table access methods.

[Phil Eaton]

Hello!

I was looking into table access methods recently and found the
existing page a bit sparse. Here's a small patch adding a little more
example code to the table access methods page.

Let me know if there's anything I can do to fix my patch up!

Cheers,
Phil

v1-0001-Add-minimal-C-example-and-SQL-registration-exampl.patch
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