samples: fully migrate basic samples to the new Sphinx extension

- Updated basic samples READMEs to use the new zephyr:code-sample::
  directive. Dropped "-sample" suffix that's not required anymore now
  that samples have their own namespace.
- Updated all references to the samples to use the :zephyr:code-sample:
  role. Checked and updated the wording of said references to account
  for the fact that samples should not have "... sample" in their name
  anymore.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>