Fixes of the image manipulation chapter

[alexander-nitsche-governikus]

Hi documentation team,

when implementing image manipulation in my TYPO3 v13 project, i had some difficulty following the TYPO3 docs on that topic:

1. The code examples did not fully reflect the keywords of the description, e.g. "columnsOverrides" was missing in the "Define a cropping configuration for a specific content element" section example.
2. Also, the examples of "Configuration per content element" and "Define a cropping configuration for a specific content element" were the same, leaving the reader a bit confused.
3. Also, the TCA override examples could do with the full array path for being a ready-to-use-example, like in other examples already.
4. The "Disable an aspect ratio" section seems to be outdated, as TCA overrides do not extend the existing defaults, but replace them completely. This might have changed, not sure, but this is what i experienced in my local instance and what can be seen in code.

Greetings
Alex