Port getting started documentation from Porcupine Wiki and update it

New screenshots
Deleted long line marker and line numbers explanation, boring/obvious/unimportant
Explained some of the most important features, including find/replace
Applied <details> to platform-specific instructions in README and linked getting started doc below them. Without the <details>, I would need to link getting-started docs before install instructions to make it visible, which would be weird.

Creates the user-doc folder discussed in #1308. The dev-doc folder already exists.

Akuli / porcupine