docs/rfcs: add RFC for fast tenant migration/failover (#5029)

## Problem

Currently we don't have a way to migrate tenants from one pageserver to
another without a risk of gap in availability.

## Summary of changes

This follows on from #4919

Migrating tenants between pageservers is essential to operating a
service
at scale, in several contexts:

1. Responding to a pageserver node failure by migrating tenants to other
pageservers
2. Balancing load and capacity across pageservers, for example when a
user expands their
   database and they need to migrate to a pageserver with more capacity.
3. Restarting pageservers for upgrades and maintenance

Currently, a tenant may migrated by attaching to a new node,
re-configuring endpoints to use the new node, and then later detaching
from the old node. This is safe once [generation
numbers](025-generation-numbers.md) are implemented, but does meet
our seamless/fast/efficient goals:

Co-authored-by: Christian Schwarz <christian@neon.tech>