Code and documentation improvements

[ingegno]

Improvements in the code, and in the github documentation.

Code:

• more profiles
• left and right handed in one code base, and changeable via button presses
• button press handling via simple code, not bitwise operations
• startup PLA for those cases where reset is happening all the time

Doc:

• info on uploading, profiles, troubleshooting on github, for easy google retrieval

[redfox05]

@ingegno @bmcage Does this still work as expected? I'm considering buying this pen, but there's lots to tweak in firmware like the default PLA material temp, and display library missing. So would be great if these changes can still be applied manually ourselves.

[bmcage]

This was for kit 1, but as you can see here, never merged.
Have not bought other kits after the one for which these changes were coded