Update docs with how to migrate from GitHub Enterprise to GitLab.com

Problem description

Customers migrating from GitHub Enterprise to GitLab.com are unable to find how to do this. Moreover, our documentation explicitly states that this is not possible:

... However, you cannot import projects from GitHub Enterprise to GitLab.com.

However, GitHub Enterprise migration to GitLab.com is possible using the GitHub Import API. This capability was added in this issue: https://gitlab.com/gitlab-org/gitlab/-/issues/228714

Proposal

Update the GitLab documentation to call out this capability.

Who can address the issue

@mjang1

Other links/references

• Slack thread (internal): https://gitlab.slack.com/archives/CLX7WMSKW/p1614961074028300