ref: add `dvc` CLI cheatsheet (maybe guide)

[Suor]

Cheatsheet is a page with a compact listing of commands or functions. It is usually:

• divided in sections and subsections by topic, optionally into columns
• single word for a term
• short description and signature/options on mouse over
• link leads to the related documentation page or page with anchor
• the less screens it fits the better

Purposes are quick navigation, discovery and glanceability. For DVC we already have group of commands, but may either group them further or list subcommands, i.e. plots show/diff/modify, or both.

Here are examples of cheatsheets for funcy python library and Clojure stdlib.