Maintenance updates

[hakonanes]

Several maintenance updates (make stuff more like in orix and kikuchipy):

• Simplify PR template
• Change Read the Docs theme to Furo: current and new
• Make changelog a ReStructuredText (rst) file, and add it to the docs via a stub file
• Update and move contributing guide to the top directory, include in docs
• Update README: move installation instructions to "Installation" page in docs, link to docs and demo repo
• Set version to 0.5.dev0, instead of next patch