istiod documentation needed for 1.5

[howardjohn]

In 1.5 we will be introducing Istiod. We need to add some documentation for this. This is still actively changing, so possible best to wait for feature freeze before writing everything, but I expect the end state to be that Galley and sidecar injector are now run inside Pilot, so we will need to clean up references to these

We will also need to document any upgrade steps required.

We will also need to document trustworthy jwt/cert signing k8s requirements