The `with-islands` island-architecture example needs documentation

[Craig Doremus]

The use-islands example is a bit of a wrinkle on Ultra that needs to be documented. Things that need an explanation include:

• The server.ts file changes compared to the basic example (and point-out the missing client.ts file)
• The IslandComponent wrapper
• The hydrationStrategy attribute and its options

I suggest you add a section to the official docs on the island-architecture option with Ultra and reference that section in the with-islands/README.md file (or just add the documentation to the readme).