[2.0.x] Add documentation for devs

Problem/Motivation

📌 [2.0.0-rc] Update documentation Active has added documentation with mkdocs. And 📌 [2.0.x] Add documentation for devs Active is currently updating it.

However, the devs chapter is still WIP: https://git.drupalcode.org/project/ui_patterns/-/tree/2.0.x/docs/3-devs

Proposed resolution

Add remaining chapters.