Update docs on working with the GitHub Importer

What does this MR do and why?

This MR updates documentation related to the GitHub importer:

• it updates the screenshot in the Select which repositories to import section;
• it describes one option on how to test importing from GitHub Enterprise;
• it seems there is no way to import a repository from GitHub Enterprise to GitLab.com using the UI. So it points out that the user should use the API for this purpose;
• it changes the explanation of the Failed import status as the import can fail after some repository entities have been imported (a bit of context - #412660 (comment 1405178422)).

Screenshots or screen recordings

How to set up and validate locally

MR acceptance checklist

• I have evaluated the MR acceptance checklist for this MR.