Add network communication prerequisite for Bitbucket Server import

What does this MR do?

Adds a prerequisite to the Bitbucket Server import documentation clarifying that network connectivity between the GitLab instance and Bitbucket Server is required for the import to succeed.

Why is this change needed?

The current prerequisites section was missing this critical requirement. Users may attempt imports without ensuring proper network connectivity between the two systems, leading to import failures.

Related issues

Closes #