add docs on how to limit associations

[LordSimal]

This is a blatant copy with some minor adjustments of https://stackoverflow.com/questions/30241975/how-to-limit-contained-associations-per-record-group/30270675#30270675

Thanks to @ndm2 to provide this kind of documentation on stackoverflow but especially recently people keep asking for this kind of feature so we should add this to the official cookbook.

I left the example the same so I don't have to adjust any code or SQL statments 😅

See https://cakephp.pfiff.me/en/orm/limit-associations.html for the generated version of this RST.

Refs: cakephp/cakephp#17192

[ndm2]

I'm not sure if putting the association examples from SO to the forefront is ideal here, as they can only limit to a single associated record, not to a "first few" ones as the introduction states.

Maybe it should rather first show Plugin usage and manual querying for multiple associated records, and solutions for single records last.

[LordSimal]

I will try to come back to this after I'm back from the US.