Add best practice of using string templates longer than 120 chracters

[LakshanWeerasinghe]

Description

$subject

Currently there is no documentation specifying how to use string templates when the character count exceeds 120. There was a spec discussion went previously and the team came to a conclusion on what the developer should do in such a scenario.

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[gimantha]

Completed with ballerina-platform/ballerina-distribution#5688