cheat sheet docs

[artalar]

Reatom has a vast ecosystem with numerous cross-links between different packages. It can be challenging to delve into a package or try to investigate a typical case because there are always many other dependencies that you should learn first. So, we need a small (one-screen) basic index page with descriptions of common cases and relevant links. It would be super nice to present it as a cheat sheet. Should we use a table, an infographic, or a diagram (we have Mermaid)?