Docs Improvement

[ssolson]

This PR will build on #9 improving the documentation and goes a bit beyond docs improvement

• the source code folder named code was renamed seat bc code is default python library
• bug fixes from Tim were pulled in
• restructured the documentation
• added dark mode

The latest PR build can be found here: https://ssolson.github.io/seat-qgis-plugin/_build/html/index.html