Update Bitbucket.org migration docs for user mapped contributions

What does this MR do?

This MR updates the docs for Bitbucket.org migration to GitLab.com for user mapped contributions.

@hchouraria and I went through the Bitbucket migration codebase and also tested on our test instance, and found out that the Bitbucket nickname that is required by our docs is now referring to Atlassian's Public name. This means that individual users would need to update their Public name to match the extern_uid that we save for the user contributions to be mapped accordingly during the project import.

This MR came about when a user (internal ZD ticket) followed our docs closely and was not able to get the user contributions mapped to the original author/assignee.

The customer followed through with the new instructions, got the users to update their Public name if necessary, and confirmed that the user mapped contributions were successful.

Screenshots of test instance

My Bitbucket username on https://bitbucket.org/account/settings/ is yongtaufoo:

I ran an API call on one of my pull requests with the username yongtaufoo:

The API response return with the Bitbucket nickname of yongtaufou (note the different spelling, with the u at the back):

yongtaufou (note the different spelling, with the u at the back) is the Public name of my Atlassian account on https://id.atlassian.com/manage-profile/profile-and-visibility:

Relevant code for the MR

https://gitlab.com/gitlab-org/gitlab/-/blob/v14.8.2-ee/lib/gitlab/bitbucket_import/importer.rb#L64

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