Remove the Contribution Guide in B.io

[praneesha]

Description:
We need to remove the Contribution Guide in B.io[1] and move it to the CONTRIBUTING.MD file in the ballerina-lang repo. It is because it is a better approach to have separate CONTRIBUTING.md files in each of the Ballerina repos with repo-specific information.

Later, we can have a Contributing Guide in B.io to explain generic information such as the contribution review process etc.

[1] https://ballerina.io/contribution-guide

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Related Issues:
Created #231 to add a Contribution Guide to the B.io later with the appropriate content.