Update GitLab Project Migration handbook page

There are several questions on multiple sections on Project migration documentation that need to be clarified/investigated further:

• https://docs.gitlab.com/ee/user/project/import/#migrate-using-the-api - why is it here, should we keep it? What’s the difference between API mentioned and those used by file-based import/export? Is it migrating project variables? I’m confused about this part. DRI @wortschi
• https://docs.gitlab.com/ee/user/project/import/#lfs-authentication - Is this still correct? Yes, this is correct ✅ DRI @wortschi
• https://docs.gitlab.com/ee/user/project/import/#project-aliases - is this still valid and relevant for import?
• Regarding https://docs.gitlab.com/ee/user/project/import/#migrate-between-two-self-managed-gitlab-instances Magda talk to PM responsible for GL backups DRI @m_frankiewicz