Better document the process around how upvotes (👍) factor into planning

Per @asilverman's comment in https://github.com/Azure/bicep/issues/15449#issuecomment-2489557312, we don't have any explicit documentation around how upvotes can affect planning, but we probably should. For example, is there a minimum number of upvotes required? Ideally, we should have a more regimented process around this that the community can follow and make sure the process is as objective as possible.

To be clear, I don't think this means that if an issue gets a specific number of upvotes it should guarantee implementation as there are always other factors at play, but it could mean that items that cross a specific threshold get an additional level of scrutiny/triage.

Also related to #12423 as having automation that can support this will be very helpful, if not critical.