Refactor upgrade docs
Refactor upgrade docs

* [x] explain semver since this upgrade cases are when this applies
* [x] links to different parts of manual
* [x] move backup section to the top
* [x] add "real" git pull example
* [x] rename install script to plain ./install
* [x] remove very old monit upgrade help

LibreTime/libretime,LibreTime/libretime,LibreTime/libretime,LibreTime/libretime,LibreTime/libretime,LibreTime/libretime