Documentation on zero downtime primary DC migration guidelines/considerations

Based on community feedback in #7817

#### Feature Description

Provide a general list of considerations for users attempting a zero downtime migration of the primary DC. This'll be a little tricky because of nuances around different configuration, so the documentation should be focused on just providing a bullet point list of things to look out for and not a step-by-step guide.

Things to consider:

- Is the user using service mesh?
- Is the user using KV?
- Is the user utilizing vault as their CA provider or the built-in CA?

#### Use Case(s)

Decommissioning primary DC without downtime.