Document how to update providers to support namespaced MRs

[negz]

What's Missing?

We released Crossplane v2 yesterday, and folks are already asking about this. We should add a guide that covers what's needed for both upjet and non-upjet providers.

Topics off the top of my head:

• Add m - e.g. m.crossplane.io to differentiate from cluster scoped stuff
• Use crossplane-runtime v2
• How to handle ProviderConfigs
• Drop deletionPolicy support

[negz]

Related: we already have https://docs.crossplane.io/v2.0/guides/implementing-safe-start/. We should cross-link it, since it'll be a good idea to implement both at once.