`newt install` and `newt sync` are deprecated

Update docs to use `newt upgrade` exclusively.
18 files changed
tree: e844213cefe4d0b24c937c005c2d35b0233774f7
  1. .gitignore
  2. Makefile
  3. README.rst
  4. docs/
  5. doxygen-mynewt-core.xml