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

> I think this should say something more like "Here is how an extension
> SQL script might create a table access method handler".

Fair point. It is referred to elsewhere [0] in docs as a "script
file", so I've done that.

> Shouldn't "mem_tableam_handler" be "my_tableam_handler"?

Sorry about that, fixed.

[0] https://www.postgresql.org/docs/current/extend-extensions.html

Phil