Draft: Split developer oncall rotations

Why is this change being made?

The goal of developer oncall is to provide expert help in mitigating incidents. However, the current scope of the product means that no single person is expert in all areas, leading to unhelpful escalations where oncall can't actually offer much help.

This MVC MR lays the groundwork for splitting oncall rotations, in order to cover specific areas with engineers who are actually effective in that area. Further iterations to refine each rotation's scope can then be applied.

Issue gitlab-org/enablement-section/discussions#48

In particular:

• introduce two oncall rotations with specific areas
• document the reasoning and tradeoffs observed
• retire pagerslack for the time being as it only supports one rotation, thus unifying weekday and weekend protocols

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