[Docs] Rework the documentation to focus it on use-cases #386

PRIMARY TASKS

• Merge Overview and Quick start
• Rewrite Quick start to focus it on use-cases
• Add How it works page
• Split theUser guide into multiple Guides per use-case
  • Dev environments
  • Pipelines
  • Apps
  • Artifacts
  • Projects
• Home page
  • Update the tagline
  • Rework the features section
• Rename Tutorials to Examples
• Restore Installation
• Update the REAMDE.md
• Fix minor TODOs (see the TODO [TASK] comments)
• Add Best practices
• FAQ

TO BE DONE SEPARATELY

• There are still a lot of major limitations for each use case. The documentation will change once they are resolved. For the details, see the TODO [MAJOR] comments in the code.
• Currently, we use "providers" as an abstraction to implement custom types of both "dev environments", "pipelines", and "apps" which is too low-level. We may need to rethink the concept of "providers" at least on the user level.