Switch to using Extension.initialize in docs and examples.
Extension.initialize is a convenience around overriding

Extension.__init__, and we want to encourage its use. This updates our

docs and the extension generator's example output to use this instead.
There's also some improvements for the formatting of the generated

extension.py file, particularly when not showing a dashboard link.

Plus, an example metadata variable with Name and Summary.

Saw that the docs looked correct.
Generated an extension and inspected the extension.py file.