doc: periodic updates to getting started guide

Updates based on discussion and changes in supported features.

- Make the guide shorter by removing content that's not relevant to
  most users who are truly just getting started, such as information
  about pre-LTS versions that did not support west, and by being more
  concise in some places.

- Decrease the number of colored boxes. At the latest TSC F2F, the
  "note / warning / note / tip" contents were identified as a
  readability problem.

- Add additional information based on new west features, like "west
  boards".

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>