Move the documentation to `doc/plugin/FocusSerial.md`, sources under
`src/kaleidoscope/plugin/` (appropriately namespaced). This is in preparation of
merging plugins into a single monorepo.
This also merges the former `UPGRADING.md` into `doc/plugin/FocusSerial.md`.
Signed-off-by: Gergely Nagy <algernon@keyboard.io>