Add context to the start of the Import API page

What does this MR do?

Related to: #322983 (closed), it's not clear which importers provide APIs, so gives the impression that some importers have an undocumented API.

Looking at https://gitlab.com/gitlab-org/gitlab/-/tree/master/lib/api, it appears there is only two:

• https://gitlab.com/gitlab-org/gitlab/-/blob/f5852b79f110754d88cf52423ae9a992958e439d/lib/api/import_bitbucket_server.rb
• https://gitlab.com/gitlab-org/gitlab/-/blob/f5852b79f110754d88cf52423ae9a992958e439d/lib/api/import_github.rb

There doesn't appear to be any in: https://gitlab.com/gitlab-org/gitlab/-/tree/master/ee/lib/api or https://gitlab.com/gitlab-org/gitlab/-/tree/master/ee/lib/ee/api (at least not following the import_*.rb filename pattern).

• Before: https://docs.gitlab.com/ee/api/import.html
• After: https://main-ee-113789.docs.gitlab-review.app/ee/api/import.html

While I was here, I've pushed anything GitHub-related in under the first GitHub section, to re-enforce that there are two main sections of the documentation.

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