Add learn examples for fork statements

[gayalkuruppu]

Description

I could not find any sources related to fork statements in ballerina site or in google search in general. What I found was some old links in search and for, foreach and for k loop related search results.

Related website/documentation area

Area/BBEs Area/LearnPages

Describe the problem(s)

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Describe your solution(s)

Improve learn pages to describe this syntax with examples.