Cherry pick `pjrt://` init method rename and doc updates (#5562)
* Change `pjrt://` init method to `xla://` (#5560)
* Update PJRT documentation for the 2.1 release (#5557)
* Update PJRT documentation for the 2.1 release
* clarify plugins
* clarify PJRT doc
* Update `pjrt://` to `xla://`
