`newt install` and `newt sync` are deprecated

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