Introduce a process to request support from Product

The goal of this MR is to introduce a process for the field to request support from Product. Currently much of these requests are taking place in Slack, and it is not ideal for either the field or the PM team.

We should standardize this process in issues, for a few key reasons:

• Easier to collaborate between the field and product, on who should join the call, what topics will be covered, etc.
• A place to capture learnings after the call is over, which can be shared amongst the team and persisted for later use
• Provide a method to easily verify requests are being responded to effectively, and not slipping through in Slack
• Provide a basis to drive insights. How often is the PM team being asked to join field calls? What are the most frequent requests? Which stages? etc.

I have tried to keep most documentation in the support request template itself, rather than in the handbook, to keep things DRY and self-documenting.

Note that along with this MR, the template should also be reviewed: https://gitlab.com/-/ide/project/gitlab-com/Product/edit/master/-/.gitlab/issue_templates/Product-Support-Request.md