docs: add documentation for column transformers

[carlos-granados]

• The "Definitions" page was too long. I have extracted the information about transformations into another page
• Added documentation for some transformers that were not documented like the row: transformer and the rowtable: transformer
• Added documentation for the new "column:" transformer

[acoulton]

Great, just one small typo / paste error in the example. Thanks for splitting this out to a separate page, much easier to find and reference!

[acoulton]

Suggested change

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	$user->setFollowersCount($userHash['followers']);
	$user->setUsername($row['name']);
	$user->setFollowersCount($row['followers']);