HelpWindow: Use offline version of user guide
HelpWindow: Use offline version of user guide

The help window accesses the user guide at:
https://se-edu.github.io/addressbook-level4/docs/UserGuide.

The user will not be able to access it if he has no internet connection.
Furthermore, the user guide may not match the version that he is using.

Let's change the user guide used by HelpWindow to an offline version.

The offline user guide and its associated files are stored at
'src/main/resources/docs', so that gradle's shadowJar deployment task
will automatically copy them to the output .jar file. It also allows
HelpWindow to access them using a simple relative filepath.

damithc/addressbook-level4,CS2103R-Eugene-Peh/addressbook-level4,se-edu/addressbook-level3,CS2103R-Eugene-Peh/addressbook-level4,damithc/addressbook-level4,CS2103R-Eugene-Peh/addressbook-level4,damithc/addressbook-level4