Update handbook Customer Training info
This clarifies https://gitlab.com/gitlab-com/support/issues/337 and depends on https://gitlab.com/gitlab-com/support/merge_requests/12 (DO NOT MERGE UNTIL THAT IS MERGED)
Essentially:
- How sales can get a training call scheduled.
- how support responds to a training call from request to completion.
Leaving the WIP Title until the support merge is complete.
Also needs a proper rebase before merge so whoever merges ping me or make sure that happens.