Add a BBE for fork statement

[Nadeeshan96]

Description

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Need to add a BBE for fork statement.
Need to explain how message passing occurs between peer workers (with and without fork statement).
https://ballerina.io/spec/lang/master/#section_7.8 says

workers created in a fork-stmt are also peers of each other. The workers created in a fork-stmt are not peers of a function's default worker nor of its named workers.

Related website/documentation area

Add/Uncomment the relevant area label out of the following.

Area/BBEs

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[Nadeeshan96]

We alsondon't have content for fork statement in learn pages also https://ballerina.io/learn/language-basics/. First we can create a BBE after verifying the fork statement behaviour, identifying the use cases. Then we can add learn page content.