Provide details about standard conditions used by Crossplane

[jbw976]

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.