Improve Cosmos docs

[tatiana]

(Written by @cmarteepants):

This ticket aims make our docs more beginner friendly. This includes have more information about the problem that Cosmos aims to solve, adding more in-depth explanations for core concepts, and reorganizing content to make features more discoverable.

ToDo's:

• Home / index: ([DOCS]: Adds Welcome Section and "What Is Cosmos" Blurb to Home Page #1251)
  • Add welcome text, and brief explanation for what Cosmos is and problem it tries to solve
  • Add a call to action to join the community
  • (Potentially) Explain doc layout
• Create a dedicated "First Steps" section with:
  • Overview: Deeper explanation for what Cosmos is, benefits, and example usage, and
  • Quick Start: Simple example that anybody can run locally (either using Astro CLI, or Airflow OSS)
• Create an Installation Section (revamp Getting Started)
• Create an Explore Concepts section to explain the different types of configs (Project, Profile, Execution, Render) and features (generating and hosting docs, only running a subset of dbt resources, scheduling options)

[cmarteepants]

From @tatiana: Another thing we may want to address is to remove the duplicate between README.md and index.rst
#1221