Get target_server ready for release

[jamesmunns]

At the moment, there are a couple of follow-on tasks from #9 to prepare for a release:

• embassy-usb v0.2 release: embassy-usb: Update version and add changelog embassy-rs/embassy#2858
  • This is to handle the breaking change of removing on of the Builder buffers for USB
• static-cell v2.x.y release: Add ConstInitCell embassy-rs/static-cell#14
  • This is to add a ConstInitCell for buffers. This is NOT needed for postcard-rpc proper, as ConstInitCell is only used in the example project.
  • If static-cell doesn't accept this PR, I plan to move this data structure to grounded instead.
• Figure out if we have any issues with unstable features and embassy_executor::task
• Figure out if we need to reorganize or rename any parts of target_server, for example allowing for other target_servers other than nusb+embassy-usb in the future.
• Docs and example pass

If you run into any issues or confusion using the features merged in #9, let me know here! I merged it so I could start using it with my conference training hardware.