Improve Cosmos docs

[tatiana]

(Written by @cmarteepants):

This ticket aims to 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:

• [x] Home / index: (#1251)
  • [x] Add welcome text, and brief explanation for what Cosmos is and problem it tries to solve
  • [x] 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 https://github.com/astronomer/astronomer-cosmos/pull/1221