[Doc] Enable less secure apps access for email notification

[prabushi]

Description:
Need to enable "less secure apps access" for the gmail account [1] to make the email notification work. It would be better if we can add that information to [2].

[1] - https://support.google.com/accounts/answer/6010255?hl=en
[2] - https://docs.wso2.com/display/Governance540/Configuring+Registry+to+Send+Emails

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