DOC: Re-structure Readme and website

[kay-ro]

Problem description:

As discussed, we should present the "How to cite" section more prominently on the Readme and the website.
Additionally, the amep getting started website looks fairly plain on a mobile device - move to visuals instead of text.

Suggested content improvement:

• include "how to cite" in the very beginning of the readme
• move "datastructures" to "Userguide" on website
• create "How to Cite" right next to "Getting Started" on website
• start list of publications where amep has been used and include on website how to cite.
• move feature table to very bottom of readme
• make schematic image of amep usage for website (visuals instead of text)
• add version selector on website (for each minor release, the most recent doc pages should be available)

[kay-ro]

• add sphinx-notfound-page for a custom 404 error page
• add sphinx-version-warning for warning when looking at older versions

todo for version 2.0.0? #104

[kay-ro]

Still to fix:

• readme-inclusion in documentation (duplication of how to cite, installation,....)
• update text for documentation front page
• simplify "getting started" with subsections installation,... (no sub-pages)
• move "Benchmarks" to User Guide
• re-organize "user guide" to simplify access to information