handbook general onboarding: Fix outdated links
Why is this change being made?
Many of the links to issue templates in the general onboarding page of the handbook were either pointing to git repos that have moved or files that had been deprecated (and no longer existed at that location). This MR corrects most of them.
What still needs to be done?
EDIT: I found more information about the support agent role: it appears it does not exist anymore, and everyone has transitioned to support engineer. Thus, I added a new commit to the MR to remove this line, and removed the draft status of this MR.
Previous "What still needs to be done" below.
There is one link that I did not find a suitable correction for:
* [Support agent onboarding issue](https://gitlab.com/gitlab-com/support/support-training/blob/master/.gitlab/issue_templates/Onboarding%20-%20GitLab.com%20Support%20Agent.md)
I see a few paths forward for fixing this:
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When I have time, I can dig through the history of the repo to find context for that link, and learn what would best replace it.
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Someone can suggest how it should be changed, or change it themselves.
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It can be removed (with a new issue opened to track replacing it?) or ignored (to just accept this MR). ~~