[RMP] Revamp Merlin "Getting Started" Documentation

[nv-alaiacano]

The goal of this RMP is to revamp the "Getting Started" section of our Merlin documentation.

The introductory tutorials are meant to cover a limited number of things:

• Introduce Merlin-specific syntax
• Show how to perform common recsys tasks (train well-known model types) in Merlin
• Introduce the concept of Systems with a basic example.

• New "welcome" page to replace README.md #1033
• Installation guide: Docker, Conda, Pip, Launchpad, Google Colab @bschifferer
• New tutorials with some "basics"
  • The >> operator, NVTabular workflow, Graph, fit/transform.
  • Training a ranking model: NVT + DLRM (or similar)
  • Training a retrieval model: NVT + TT (or similar)
  • Creating a basic System: Fetch Features -> NVT -> Ranking -> SampleSoftmax. (Since there's no retrieval model, input can be something like {user_id: u, item_ids: [a, b, c, d]}) @nv-alaiacano