Remove training module from the handbook

Why is this change being made?

From slack: https://gitlab.slack.com/archives/CCBJYEWAW/p1638510371039200?thread_ts=1638505988.031300&cid=CCBJYEWAW

It seems we have two sources of truth (one in the handbook with this MR, and one in the support training template). They appear to have the same content as well.

I feel we should remove the handbook page (moving any missing details into the support-training template), and concentrate on using the support training template, since that has checklists making it easier for the support engineer to follow, and is where all the other support-training modules are located.

EDIT: I have checked, and in fact the handbook page contains out of date information. It does not contain anything we need to move to the support-training module

Author Checklist

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• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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