Provide details about standard conditions used by Crossplane

### URL

https://docs.crossplane.io/latest/concepts/managed-resources/#conditions

### What's Wrong?

Discussed a bit in this PR: https://github.com/crossplane/crossplane/pull/5453#discussion_r1562242295

Our docs focus on the possible specific reasons for each condition, but don't seem to focus on the high level conditions themselves, e.g. `Ready` and `Synced`. What are their intent and semantics, what scenarios do they cover?

@negz quick write-up that is useful:

> Today we have the following status conditions:
>
> * Ready - The external resource is ready to use, (where applicable) according to its API.
> * Synced - The managed resource's desired state has been successfully applied to the external resource.